Not 100% sure but the normal MK5 GTI uses a K03 turbo but the ED30 uses the K04.
I have a VAG 2.0T and it's a very tuneable and reliable engine but it sounds boring. I nearly got a MK5 R32 rather than the S3 but the fuel, tax and tune-ability made me change my mind.
Correct. In the Mk5 the 2.0T came with a k03 in the GTI and a k04 in the ED30. The k04 is the turbo to have, as it can hit / break the 300 BHP barrier with relative ease. It's the same turbo as the S3 and the Leon Cupra.
But the Mk1 TT, as mentioned, uses the 1.8T engine. And the 225 BHP variant uses the k04 turbo, same as the S3 and Leon Cupra R. While still a great, tuneable engine, it kinda peaks out at around 270-280 BHP without a turbo upgrade. Not that that's not enough mind.
But that's all an aside to the original question to be fair. And in answer to that, I couldn't agree more with what you have said. The fuel consumption and tune-ability of the 2.0T engine would always sway me towards that engine. The tax is neither here nor there though, as it is a small cost in comparison to all other costs basically. With the V6, you are swapping tune-ability, a sportier ride and fuel consumption for smoothness and note. For some, that is obviously worth it. For me, I'd prefer the power personally.
The Audi V8, as found in the S4 for example, is a much nicer engine for that. It is only marginally less economical than the V6 in the R32 (like only a couple mpg difference), has proper 4wd and an extra 80-90 BHP. It feels distinctly faster, and, despite it's bigger size, I felt it drove sportier, more "on edge". And prices are similar between mk5 R32 and B7 S4.
If I was looking for a car that gave me note for driving about in, and was happy averaging low-mid 20's, I'd EASILY pick the S4 over the R32 every day of the week. That is why I bought a B6 S4 instead of a mk4 R32 (when swapping out my V6 4Motion).