Not only do Nelson and Jett engines have superior piston to sleeve fits, they have 3 other very important things going for them as well:
1. 17mm crankshafts (2mm bigger than your average .40)
2. Tuned mufflers.
3. Very high compression ratios (15:1 measured full stroke)

The only other .40 size engines I know of that have 17mm cranks are the Rossi (the first to do so) and the K&B .48. But they have less compression and more conservative port timing.

Does anyone remember when the Rossi .40 first came out in 1986? It changed everything. It's 17mm crank, relatively high compression (about 11:1 full stroke), and sorta tuned muffler paved the way for the Nelson .40. Henry Nelson took all those ideas and pushed them to the limit when he released his Q-500 engine in 1991.