You will definitely see a difference with a 3" exhaust. I tried the 2.5" FRPP X pipe with the stock exhaust. Sounded bad and slowed the car down as well.
Just spoke with a rep at LMR....
http://www.latemodelrestoration.com/item/M8005M8/2015-Mustang-GT-Ford-Racing-High-Flow-Radiator-M-8005-M8
The rep said this is the performance pack radiator. If that's the case, Tasca has them for $450. I sourced one locally for $505.
On3 kits are cheap and will work but they generally do not fit. They usually take some modifications to make them fit properly.
I personally have no experience with them because of their reputation for poor quality.
I prefer turbos over any supercharger any day of the week. Both kits will get the job done though.
FWIW I have the 55's and the spool up isn't that fast. Don't get me wrong, it spools but there is lag.