I put a twist on my yfz recently, but i never had too much experience with the twist, and found it's a much bigger change when you've been riding for a
long time with a thumb lever. i lost a lot of the confidence with jumps, wheelieing over obstacles, etc.
i put the thumb throttle back on but kept the TORS bypass in, and took the thumb assembly apart and removed the spring. that way i'm only fighting one spring, the over all throttle stiffness is the same as the 660r. much lighter, and when i took it for another ride, i felt like a whole new machine. i gained all that confidence back, and my arm didn't tire nearly as fast.
as for that thumb assembly spring removel, it's safe since the piece it attaches to is only there to operate the TORS switch, and i disabled that system anyways.(I have previous experience with the banshee, so I'm NOT
a big fan of the TORS system. its only there to placate Yamaha's corporate lawyers)