Well, first thing is - any load that doesn't exceed the established SAAMI pressures (not +P, but "ordinary" pressures0 for a .357 Magnum will be OK. The guns are proofed for the round marked as what they are chambered for, so any .357 Magnum round or load (other than loads of a +P nature) will be OK.
Reason for keeping lead bullets at or below 900 FPS is faster loads are likely to lead the bore heavily. If you want faster lead bullets, use gas checked examples and cast them REAL hard. And still expect (or at least don't be surprised by) considerable leading. Might even strip (or jump if you prefer that terminology) the rifling (as in the bullets strip and come out land diameter and not spinning normally), with adverse effects omn accuracy.
Ain't got a clue about not using HPs in the Uberti 1873 clones. Never heard that and can't think of a single reason for it to be a problem.
Oh - best bullets are the ones that turn out to work best for what you are doing, or (probably a better way to decide) the ones your gun likes best and shoots more accurately. Revolvers shooting lead bullets are often like 22 rim fires and have preferences - this is why people do laod development, and it can be real frustrating until the right combo of bullet, powder and velocity is found for the particular gun.