Hi Flo cats do have a history of throwing SES lights. I went with the Goerlich, because my exhaust guy said that the Magnaflow do through SES lights on the Bonneville. My cat has a 3" in and out, and I could feel the difference (I had the entire exhaust done from the down pipe back).
what is the stock pipe size and where do i buy that cat you got from?
Google Goerlich, if yo don't have a local muffler shop you can buy from. We've had some discussion here before and I think it'* probably 2.5. If you're going with a hi-flow cat to get better response and in prep for more moding, you should consider going 3" from the down pipe to the muffler or Y piple.