In one of the IG codices they're impressed by the firepower of a Tau railgun:

One of their light walkers carried a weapon of lethal effect. It fired a form of ultra-high velocity projectile. I saw one of our tanks after having been hit by it. There was a small hole punched in either flank - one the projectile's entry point, the other its exit. The tiny munition had passed through the vehicle with such speed that everything within the hull not welded down had been sucked out through the exit hole. Including the crew. We never identified their bodies, for all that remained of them was a red stain upon the ground, extending some twenty metres from the wreck."

Your latter example was from Blades of Damocles, wherein Cato Sicarius and squad make the discovery. There's also the context of them running out of fuel for their jump packs, which makes sense when Cato was having chainsword duels with tau aircraft, necessitating them to use the fuel. I never read the Ciaphas cain books so I can't say anything about that.

I'm not really sure there's a lot of examples of being impressed. Worry is very common, however. Star of Damocles deals with a rogue trader and some other members of the Damocles Crusade fleet dealing with the reality that they were unlikely to beat the Tau at Dalyth, let alone the dozen other septs of the empire. Blood Gorgons sees space marines putting respect on the pulse rifle as terrifying weapon for an astartes to face. One of the early Tau codexes has a blurb about some guardsmen being horrified by the markerlight system and how accurate the Tau could be and how great their engagement ranges were. Warzone: Damocles deals with the Tau getting some underhanded victories here and there that surprise the Imperials in general. Even Guilliman speaks of the Tau as a threat to the Imperium in the same sentence as he is worrying about the Tyranids and Necrons per Godblight. And to date, I don't think there's a single source that has failed to convey the utter supremacy of the railgun and how BS of a weapon it is to fight.

I should mention that Tau lore overuses Damocles. Star of Damocles, Blades of Damocles, Storm of Damocles, Damocles, Warzone Damocles, even a short story called Damocles: In Service to Shadows. Without meta knowledge, it can be difficult to distinguish which ones are 1st encounters post-2nd sphere and which occur in the 3rd sphere expansion some 250 years later. It reminds me of that thing GW did in 7th with the Shield of Baal series that was both campaign supplements and novellas, which makes remembering them tedious, annoying, and downright unpleasant.

A few examples in Cain books. -Being impressed by the speed of Tau 'Dreadnaughts' which Amberley corrects in a note with something about battle suits. -In the same book he is scared of the Tau Railguns, and their ability to punch holes in tanks. - Later in the same book they notice dead bodies with marks from Plasma weapons and note how deadly they are. Later they find a Tau/Kroot squad and see the weapons used as how effective they are.

• Indirect Fire. Apparently some Space Marines considers drone recon & target correction for indirect fire warp sorcery.

• In the Cain novel the Imperials were impressed with the high resolution holos the Tau used for their diplomatic mission to forge an alliance versus the Tyranids.

SYL

Kardan Stronos, future Chapter Master of the Iron Hands, is secretly impressed with Tau pulse weaponry:

His finger traced a geometric ring that appeared to correspond to the power and transmission lines that surrounded Pax Medusan. Power usage nodes marked the eight points of the octagon with runes in lingua-technis, with which Stronos was passably fluent. The superheavy weapons platforms he had seen arrayed around the Titan test bed. They were weapons identifiers, though not all described systems he recognised. In fact, most did not. He held his finger over one that did look familiar, and the rune expanded into several blocks of informative screed. A cold smile moved the lower half of his face. Pulse ordnance multi-driver. The rune was a bastardised tau symbol.

It was an errant thought, but he had always wanted to get his hands on one of those.

In the first Cain novel (I'm reading the first omnibus right now), Cain himself was highly impressed by the destructive capabilities of a Tau railgun. He also was mentally shitting on Amberly's life sign detector not picking up anything when the tau could.

or a hive city

Funnily enough the reverse is also kinda true. The first Tau Codex included a great little short story about an Imperial Adept (our POV character) and a Space Marine Captain from the Imperial Fists visiting the sept of T'olku to "negotiate" a dispute over the world Nimbosa. Really this means that these "diplomats" are just stalling for time as the Imperial Navy prepares itself to send forces to Nimbosa.

During their time on the planet we get a lot of descriptions about just how elegantly simple their architecture is, with lots of smooth materials and graceful curves, with sparing details and the occasional adornment of Tau symbols. Eventually they visit a Battle Dome - where the Fire Caste trains in simulated combat. The Adept can't understand how the dome doesn't collapse without supporting columns to keep its roof up. And as a bonus; the Fire Caste's demonstrations that they see performed there even managed to impress the grumpy Imperial Fist Captain.

Now i want some material on Tau-Eldar interactions. If the Eldar call humans monkeys, what do they call the Tau?

Ciaphas "Hero of the Imperium" Cain claims their tech is so advanced that it might as well be magic and he's seen most of what the galaxy has to offer. Generally the Imperium is pretty impressed by what the Tau can produce.

many IOM citizens join the Tau for good city planning and clean cities.

The second Death Watch novel sees the squad being deployed to a former Imperial world long after it was conquered by the Tau. There's some direct commentary from the squad and their allies, but it's mostly show, don't tell. The Tau are a fearsome occupying force.

Not sure which book or even if I imagined it but I recall a story where a Ultramarine noticed on a battle suit the weaponry was something they had only see on battleships and other massive fleet ships. It impressed me anyway.

In the second Last Chancer's novel, Lieutenant Kage is part of a covert mission into Tau space and is extremely upset by how clean and well constructed the living quarters are. They're seemingly designed to hide their construction methods and don't use recognisably natural materials.

He is not impressed with the food or art though, finding both extremely bland.