I am sure that if you buy a sports cat, the pipe will incorporate the other bit you have. That is certainly the case with my downpipe and others that I know of. If you fit the sports cat, you will need to remap. I think you will find quite a difference. PM STi_Bandit as he has recently done a similar thing with his STi BlobEye. Be warned though, that a remap may require other mods to cope with the increased power; injectors and fuel pump most probably. Before acquiring the parts for the map, you will need to contact AndyF for more details, he will tel you exactly what you need for the power you have in mind.
hope that helps!
J