I'm also a fan of the Speed carb. Until recently I was running it on a Flying Horse 48 cc.
I compared the sizes a NT vs Speed Carb.
NT carb........ = 13.74 mm
Speed Carb... = 14.78 mm
I just changed motors in my bike. I kept the same speed carb.
IMO the speed carb is a better overal carb than a NT. The NT seems like it has a little better bottom end throttle response, but it's a lot more touchy about 4 stroking on the top end.
With the Speed carb, the motor starts easier, and idles a lot better. Any 4 stroking is greatly reduced, or eliminated.
This is my new Flying Horse 80 with Speed carb