I would try to prop where you can get closer to 5000 rpm or above. You should see a considerable increase in speed. I wouldn't increase your boost on stock motors either. Try playing with props, you aren't even close to your power band yet. Did you try three blade props? If you can get on plane with mirages you will definantly be faster. My 30' Kachina with a single 502 mag mpi, headers and Vortech charger (with the fuel kit and Ecm reprogrammed by AZSM) does 75 with a 28 pitch bravo @ 5100 rpm or about the same speed with a 26 pitch at about 5400 rpm with a better hole shot. I usually run the 28 so I dont rev so high but I like the feel of the 26.
Seems to me you would run a whole lot faster if you could get into the right rpm range with a better prop choice. Granted 26's may seem like you wont run as fast as I would but you should have alot less slippage with twin motors, that should increase your speed.