The battery is only capable of delivering so much power, because of the cells used to make it, and due to the wire size in use. The Bolton kits/KT controller is able to pass whatever the battery can supply in this case. It's not going to make the battery more capable than it is!
If you are seeing a 5v sag, either the battery is in a fairly low charge state (below 46-47v?) or you are trying to pull WAY too much power from it.
I'm using a 1000+ watt MAC 12t motor, and have set the max amperage I can pull at about 1200w max (actually just less, I see 1147 or something like that on the display). This seems to make everybody happy (controller, battery, and motor), and I have all the power I need to climb any hill I've pointed at without using PAS 4 or 5. 3 year old battery still has a an easy range of 35 miles with me on it (I'm 315lbs) and well over 50 with the wife on it.