The factory 240 turbo camshaft used a 405 or 531 head, as the standard head had to be retained for homologation purposes. A certain amount of porting/polishing was allowed, though, and the cylinder heads were modified by Grottis. A full-race cam was fitted, along with lightweight forged pistons, crank and rods. Combined with the Garrett T3/T4 turbo running over 20psi, a huge intercooler and water injection, that's easily enough to see the power figures quoted.
Some Group A cars were also fitted with an early form of traction control which could cut fuel to 2 cylinders when it sensed wheelspin. the system was tested, and even fitted to some racing cars, but reportedly never used in race conditions due to the fear of teething problems.
Source.