I had a Magnaflow cat but didn't get the direct fit. I got mine from Summit and had it installed for cheaper than that direct fit one. Not a lot cheaper but it was a little cheaper and the local guy did a awesome job on the install. I didn't even have to touch it where as with that one you have to install it yourself. While not hard may be a pain.

 

Sounds like you may be loading up on fuel. Without a scanner and/or gas analyzer, it's gonna be hard to determine, but if I were you I'd go cruise it at the speed it acts up most for a few miles, then drive it home as gently as possible. Pull a plug and see what it looks like.

Possible culprits of rich running condition are:

-O2S reading incorrectly
-MAP reading incorrectly
-CTS reading too low
-Fuel pressure too high

Vacuum leak is a possibilty, but it will present mostly at idle and regardless of engine temp.

It's not the cat. Clogged cats restrict airflow, which means the engine will be most affected at RPM under load.