I would say 3x at most.

There isn't any disadvantages with mining on water.  In fact you can overclock and get way more Mhash with low noise and little heat.  My 6990 has been mining forever @ 990 MHZ and is currently at 50C.  It gets 850 m/hash.   Can up voltage and easily hit 1025.

Unfortunately water cooling is not easy, although I think it may be easier with mining.  As you have no case you won't have to worry about shit not fitting and buying the wrong stuff.

I'd hop over to OCN and review the water cooling threads.