i did mine 2 week ago and from the track testing and but feel id say about 5 horsepower if youre old cat is ok and youre exhaust will be a bit louder and gutting youre cat will hurt performance and give a code . but be sure to get the high flow one

Out of the high flow Cats we have to choose from which would be considerd the highest flowing out of the group?

Don't know that there is highest flowing one or that it really matters. The Magnaflow I have in my signature worked and fit great for me. Just make sure you measure your pipe diameter ahead of time so you know which one will fit.

You dont want "gutted out", you want high flow.

When I put in a high-flow cat in my 93, it made a difference in hard-acceleration. Mind you it was not mind-blowing difference, but it was noticable.

This will depend on your mods. I changed my cat only when I had a 3.4, pems and a gutted airbox. Honestly..all I noticed was a louder tone. Overall, I wasn't flowing enough to need a high flow at that time.

If your cat is clogged or bad, yes.....high flow it.

Thanks for the advice guys. Yup the reason I was was asking is because when I got my car it had no cat, and since I am planning to start driving and modding it this mid winter I want to get it installed along with my new door.