I think you could keep the cat as it looks like the pipe diameter starts to reduce just after it. There are some 3" milltek down pipes for sale on ebay and they are pictured. Check them out you will see what i mean.
I think you will be ok for cells with a decat. your stage 2+? If not im sure apr will have a map for decat for little or no cost. The only
problem you will have then will be MOT!!!