Generally speaking it's the amps that give you the torque and the volts gives you the speed. If all three drivers pulled 4 amps then you'd need 12, but it's normally understood that this doesn't happen, so your 10 amp supply seems OK. But you could use more volts - up to 56 - and you'd then get more speed.
I normally think of feed rate on a per-minute basis, so you're getting 60 inches per minute. It depends how big your machine is - mine is tiny, so I'm happy with a lot less than that.