If you run the same pressures, the improved provides another 50 fps, if that. Those that claim outrageous velocities achieve them by running high pressures, not by some wonder of the improved design. The improved has more expensive dies, you have to fire form brass, and the re-sale of the gun is much less than the std chamber.
I had a 35 whelen AI, and would never go that route again. If your really want more power, then use a noteably larger case, ie 358 Norma or 358 STA. I went with a 350 Rigby when I had my AI re-chambered, and prefer the larger case. I would also be happy with the std whelen case.