I had a factory 300tdi auto RRC. Performance was at best, sluggish, but the fuel economy was the biggest issue. It wasn’t a good marriage - I wouldn’t recommend it.
My 200tdi series, with 5 speed box by comparison, drives very well, and delivers literally double the MPG.
with hindsight the RRC was no more economical than a V8 Petrol one, but a Petrol one would have been a hell of a lot nicer to drive!
the power band for the TDI is just too narrow.