As mike have said, its all up to the rest of the engine, but simple put is because of the flow caracteristics of the cossie head then it doesent need a wild exhaust cam because the flow in the exhaust chanals is so much better then the inlet chanels that you can use a milder cam to get the same power up high in the revs, but the same time if u want some low down power you have to "hold" on the exhaust gasses to get the speed/temp up so the engine uses the gases better.
some sweedes have actually gotten over 600bhp out of an cossie with std exhaust cam, but if u upgrade it on the same engine to a bd-14 u loose a little low revs but get a really big kick over 4000rpm.
Basicly it“s up to the person to find out if he wants top end power or midrange, then the head porting and cams can be specced after that.