Here's a good rule of thumb. If you want to make 300hp, you have to run right at 450cfm of air thru the motor, 1:1.5 is the accepted rough ratio.

A 12 pushes somewhere around 350cfm @ 15psi, or 230-235hp, at a pretty high boost where you'll need water meth injection or race gas or e85 or a ton of timing retard to avoid detonation. It has a very early spool up and is typically at 7+psi by 2600 in a stock motor behind an a/t with the loose stall coverter, rated somewhere between 2200-2400rpms. I used to see a full 10psi at 3000. 15psi is what finall killed my 150k motor (see what I mean about boost addiction?). It runs out of breath at around 5200.

A 14 pushes 465cfm at 15psi. 300+hp. The spool is almost as fast as a 12. I drove a starquest both before and after a 14 transplant. You could still lug it around with good pull at sub 3000 engine speeds, and it was a lot faster in the midrange, running out of air at 6000+.

I think if you run a heavily laden swb truck, the 12 is a better choice for most of us. If you like to humiliate ricers, the 14 is the tool of choice. The 12 also gives you room to feed the addiction in less expensive steps. A new 14 can be had for under a grand, easy. Then a small 16, then a big 16, then the 17 and 18 and 20, and then you jump into the Garret ball bearing GT series and a multiport intake and a diesel intercooler on the roof....

Addictive...