Stock for stock, not much comparison as the 5.0 will rape the 2.3 out of the hole. BUT, get yourself a manual boost controller and turn the boost up to about 17psi and it's a different story. My Merkur will slaughter a stock 5.0 once it gets moving. It's still a slug off the line but once it gets to about 3k RPM it snaps. On the highway it wouldn't even be a challenge....it'll go 65-100 almost instantly and i don't even have to downshift. All i've done to it is put a K+N on it, a small notebook sized Saab intercooler (most ghetto IC install ever) and set the boost at 17psi. It even has a 2 inch exhaust all the way back...i can't wait to get a 3'' exhaust and a ported head and intake.
Here's a really quick 2.3 Mustang without a ton of money dumped in it
Home of Turbo 4 Cylinder Mustang