Bugger,
I have no idea at what velocity the 350 TSX performs best, and probably nobody else does either, because there's really no upper limit for monolithics. At the other end, a good general rule for any expanding big game bullet is a minimum velocity close to 2000 fps.
Yes, the .416 Rigby can be safely handloaded to very close to .416 Weatherby velocity, because powder capacity is very similar. I ran some experiments with my 25-inch barreled CZ several years ago, using some formulas for calculating internal ballistics, and got just under 2800 fps with 350-grain bullets and around 2650 with 400's. Those results were published in HANDLOADER and, later, in my BIG BOOK OF GUN GACK.
The formula for working up reduced loads with 5744 is:
1) Fill a case with 5744 to approximately where the base of the bullet would be.
2) Weigh that powder charge, then use 40% of that as a starting load.
3) Consider 48% maximum with cast bullets, and 60-65% maximum with jacketed bullets.
