Have used Free Wheel with my rigid chair on asphalt trail with up and down hills. Worked great, fun ride. Also used it when my chair was hooked up to my ZX-1. A great device. There used to be hardware to hook it to back of wheelchair when not in use. Don't know if that's still available.

For downright speed, have you tried a handcycle, (low to the ground, with 3 wheels)? I got one at age 55 for recreation not racing - after using a race chair for 12 years. The issue there is transporting it to a paved trail - fitting it into your vehicle - if you don't have paved access from your home.