Looks like a 160mm rear rotor there Pedalex. Man thats close. Take the chopper for a medium length ride and then go over the entire wireing with your hand feeling for warm spots. These would indicate resistance. Redo these weak areas with larger wire, or higher amp connectors and make sure the tires are pumped up to the max tire pressure. If that doesn't help, I feel you will need more battery power. These choppers are a heavy lot, and with an adult on top of that, you will need all the help you can get. Also, I read that Conhismotors aren't the most efficient out there. They appear to be quite power hungry, from what I read.
Remember, the larger wheel on the GT will translate to higher ground speed with everything else being equal. I figured 22 mph was good speed for a 20 inch wheel. The high amp draw was the flag for me.
Upgrade that motor to a 1000 watt unit, and hit that with 30AH of LIPO and tell us what you get!
Hit Choppers US for brake cables and handles. http://www.choppersus.com/store/product ... r-Blk-Slv/
I would go for a larger rotor first though. I have the stock 180 mm. Tektro setup on the Spoiler, and all I need is one finger to stop it.