I rock out in my 02 Jetta TDI w/ 300k+ on the stock engine, clutch & trans. It has a rebuilt head & inj pump on it as well as slightly larger nozzles. Peppy but def no tuned Dmax.
I get 40mpg no matter what I do. Best has been 42 & worst has been 39. Most get better mileage I think but it's still hard to complain about 40+ mpg.
I do have a rant though... Everyone wants to rag on domestic cars compared to foreign, especially German. Well, I can promise you none of the GM vehicles I've ever been in besides my plain jane 88 s10 and an older square body truck ever had more interior rattles n noises than my car. There's more in-cabin road noise, in-cabin engine noise, rattles, broken junk latches, crappy plastic etc in my Jetta & it's a fully loaded top of the line model. So, NO, IMO they are NOT superior to most domestics that I've been in. Especially of similar vintage/model yr. that's completely false testimony IMO.
Can't knock the little engine though.
My next one I'd like to think will be a chevy cruze diesel though.