The car running 670ps was a project car with a big turbo conversion which was tuned by MRC and various other tuning goodies added. However, it was running standard internals which is a bit of a gamble when taking the engine above 600ps (the recommended start point for uprated pistons and rods).
If you want to see what can be acheived with the iron block engine and the latest developments with the new alloy engine, take a look at Iroz Motorsport. Some of their conversions go to over 850+ps
http://www.irozmotorsport.com/
Thank you very much, will have a read in a minute! Just looking through peoples posts to see what mods people have done! Hoping to come across an aftermarket wheel topic