Our lean burn head have a continuation on the flat bit of the 'D' to create a 'heart shape'. I am not sure this is a restriction to be honest but any thin or sharp edges need to be removed to reduce rick of detonation....the only advantage of having the smaller combustion chamber from what I can tell, is it creates a 'squish' area (the flat bit on the 'D')to move the air/fuel mix closer to the spark plug. We generally run a lower compression ratio and wind the ignition advance up with more boost so the engine runs cooler. The only disadvantage of this is the slight loss of off boost drivability as there is a small torque defecit. Once it'son boost, it's a different story. All the RS Turbo heads where a complete circular dome type (same as mine) and not the lean burn. This obviously drops the compression ratio as a big lump of metal is missing compared to the lean burn. Standard RST compression ratio is 8.3:1. My own car is 7.5:1.

I believe the lean burn heads have also been used to create some decent power but I don't know who's cars they are on....

If I recall correctly, the most we have seen using a standard (except a bit of manifold to head port matching) RST exhaust manifold is around 315-330 bhp at the wheels from the 1600cc engine. This was not using a T3 turbo though (a T03/4 was used).