You should add a field for "Maximum amount" for product volume discounts. That way, you could create a structure to allow for graduated discount levels. Example: 3-5 items 5%, 6-10 7%, and 11+ 10%, etc.

Or maybe it could also (or maybe this is another feature?) allow for blocks of discounts. So we could implement "Buy 3 Get 1 Free" kind of deals. That way, you wouldn't have to use the Volume Discounts now to give the 4th, 5th, and every other subsequent item a 25% discount, instead of just every 4th item a 100%. Which would be the same price in the end if the customer bought exactly in increments of 4... But if they bought 7, then those last 3 would have a discount that shouldn't.