Tim,
The 21 inch front wheel will add around 1 inch of trail to a VTX1800 with the KMPS kit. Your trail right now is around 4 inches with your 18 inch front wheel, KMPS kit and 4 inch extensions. TThe 21 inch front wheel will make your steering a little heavier at parking lot speeds, and even more solid at interstate speeds. Remember, the stock VTX1800 has 5.8 inches of trail, so you will still be under stock. The KMPS kit is designed to be in the safe zone with an 18 inch front wheel OR a 21 inch front wheel.
Matt