It depends quite a bit on how rough you are with it when driving. A 12v motor and an 18v drill motor perform essentially identically at their nominal voltages; the 18v motor may have slightly superior torque. However running any overvolted motor into a drill gearbox is asking for trouble. We ran 12v motors at 5s in Tormenta 2, and we were killing gearboxes and thus frying motors left and right. To try and fight this we moved to 18v motors at 5s, we figured the lack of overvolting would reduce the kick of the motors and so reduce some of the stresses on the gearboxes. That uh... didn't go to plan. We were killing just as many as before. Not entirely sure how/why.

I think there's the simple underlying issue that, drill gearboxes, as they are, aren't good enough. You must replace all gears with metal ones (can be bought at gimson), and I'd recommend trying to keep a feather finger on the throttle if you don't want the carrier pins, on the second stage of the gearbox, to come loose and jam the gearbox. The latter failure is what plagues T2. There are other failure points too.

Basically, you can use them, but they will require lots of maintenance unless you're very kind to them. That's our experience. I would not recommend overvolting any motor combination with these gearboxes, as the additional torque produced by an overvolted motor makes preventing the gearbox insides from eating themselves all the harder.

There are ways you can toughen that second stage carrier plate/output shaft combo, and thus make overvolting a possibility. A straightforward solution has yet to arise, though, as far as I know.

tldr; I'd avoid the overvolting if possible, drive with care and you *should* be fine.

As for amp draws, the 18v motors at 5s in T2 pull about 1-1.5ah in a fight. That's four of them, mind.