guy at MOT can recognise if the cat is working or not (dummy), as the emmision results are much higher than should be.. (acc to manual concent of HC with cat should be less than 90ppm whereas without cat the amount of ppm is almost ten times higher... the same for carbon monooxide..)
on cars with electronic injection/ignition there should be something very wrong (worn engine-valve seats, piston rings, leaking injectors, misfiring ignition), or should be highly modified to exceed emmision levels...which aerts the guy at mot station
in czech you should have complete exhaust system, and exhaust gases are analyzed "online"... so there is almost no way how to fake it
thats why I dealt with the cat
here is rought guide what size of body and density of substrate suits to certain bhp levels:
but in case of high amount of exhaust gases and big cams I would preffer 100CPSI substrate only - those engines are higly sensitiveto small increase of backpressure..
here is also some reading (exhasut manufacturer for Porsche) :http://europipe.be/exhausts.html