WOW???
Guess I am totally out of the loop on this?
I didn't even realize that beretta ever chambered the Cougar in 45 acp, much less in the compact??
I have shot both the (new) Stoeger cougar in 45 and the PX4 standard in 40 and both are excellent weapons.
actually i own a stoeger cougar in 9mm and a beretta PX4SC in 9mm, same story super good weapons.
As they are basically the same design (roatating barrel lock up) and you are speaking of the same caliber(although in a compact the 45 has a fairly low capacity I would imagine) then the only real difference that i can see is the Cougar is an all metal gun so heavier and the PX4 is a poly frame and so lighter.
I think that you can get a new stoeger cougar in 45 for about that 450 price, and i know that you can get the 40 caliber cougar at that or cheaper, but not a compact.
can't help you with the barrel swap etc however.
The magazines are not interchangable i can almost guarantee you of that.
hope this has helped a bit.