e46 M3 sports cats

[Andy70]

Hi all I'm looking at re-mapping my car and considering putting on some 200 cell sports cats, the BMW specialist I use recon they will get through an MOT but would struggle, is this correct? also, what gains could I expect with these if there is no problems? tuning companies recon with just a re-map Id get 20bhp but not sure what extra power and torque id get with upgraded cats. Also why do the prices vary so much, some companies are asking between £250 and £350, others I've seen want closer to a grand and they all look very similar.