There should be no real need for wraps or additives when the cat is in downpipe close to turbo, its when you have to fit them further back in certain section for example that temps start to drop..
I wouldnt go putting Dipotane in the fuel as its an octane killer..
Sports cats seem to be the job as far as im concerned, im faced myself with having to take a downpipe of to refit a cat downpipe for test and then swap back again, and its a pain in the hole and thats when everything goes right for you and no bolts ring off etc etc..
Ive seen reported figures of only @ 1% power loss when using a high flow cat over a decat so dont think power loss is a concern