The biggest con like _K_E_P_ said is that some skimmers want to see a constant level of input. You can get around this by having a valve before the skimmer to regulate flow.
You will probably need to throttle back on your return pump to match what you are putting through the skimmer. Your overflow wont back up, it might run too slow and the tank will overflow! You will also want to modify it to recirculate the water in the skimmer, not suck in sump water, like it does now.
My G2 is direct fed, and valved down a little. It has more than doubled its production since the mod. I diverted the excess flow to a refugium, but in general I am running a really slow turnover in my sump (about 450 GPH).