I just found this new web page which gives pics and specs for the racing versions of the 505 turbo:
It is on this website which has been around for a while and has alot of other interesting stuff on 505s and 4X4 50s etc:
If you want to read it in English, copy the URL, then go to Altavista translations (http://babelfish.altavista.com/translate.dyn), paste the URL in the appropriate box, make sure you select "French to English" and then click translate. The grammar is a little bit rough, but it does the job.
Apparently the top racing version of the 505 turbo had 540hp and 58kgm (approx 568Nm)of torque, running on 1.9 bar of boost (which is about 28.5 PSI). Pretty awesome stuff. Even the production racing version had 240hp and 33.6kgm (approx 329Nm) of torque.