I think it's a great multi channel amp. Unfortunately... you are losing performance (power) for form factor. Why not look into a good 4 channel amp pushing 150 watts @ 2 ohms and a good 3-600 watt monoblock amp fo the subs?
Have you looked into Hifonics? They make excellent car amplifiers and the price is right. Check out the Zeus series. You could easily buy 2 amps for the same price as your paying for that one underpowered Infinity.
http://www.sonicelectronix.com/item_17652_Hifonics+ZXi+6410.html