M3 GT4 is a
race car designed by BMW Motorsports to compete in the FIA GT4 series, which is a series designed for amateur racing drivers to run cost-effective race cars.
Therefore the GT4 rule book only allows
minimal changes from series production M3 components, and engine modification is generally not allowed. The cars even run catalytic converters.
Nonetheless, for the parts that are modified, i.e. brakes, suspension, clutch disc, they are racing parts and
need to be rebuilt on a regular basis, so it would not make sense to run these on the street.
The M3 GTS is a
street-legal track car designed by M GmbH. It is not designed to any specific racing series, so everything is basically a free-for-all for M. This is why BMW can increase the engine displacement, add aero, etc.
Performance wise the GT4, being a stripped out race car (3150 vs. ~3400 lb), with race suspension (~twice as stiff as GTS), actual slick tires vs. street legal tires, I would image it would be faster despite being 30 hp down.
http://www.m3post.com/forums/showthr...&highlight=gt4
http://www.m3post.com/forums/showthr...&highlight=gts