I had a PWM on the 12v Rule livewell pump that I was using. It didn't make a difference in proformance at all. Just made the nosie of the pump more anoying. And the Jabsco runs so quiet you can't here it and the proformance is great without it. I still have the PWM and I'm thinking about using it to run my fans.
edit: You could AC motor speed control like this one found here
http://www.hobbytron.net/G-119P.html and if yourgood with electronics you can build a thermo sensor to adjust the speed instend of using the pot.