audi managed to make there diesel rev and make power up to 5500 rpms
if you have an 8v hdi then you can get a piper cam for it, and also a bigger turbo might help things and running an lpg mix and some bp ultimate fuel
I have seen people make useful power to 6k with them might be worth a look if you are that way inclined
http://www.allardmotorcompany.com/filestore/list/documents/Allard-A3-TDI-quattro.pdf
it can be done,
the bloke goes by the name of "devonmikeyboy" on various vag and diesel forums