"I'm wondering what sort of velocities these can be used at without causing leading of the barrel. I understand that correct projectile size has an effect on the amount of leading caused as well."
That's virtually impossible to answer with any validity but a properly hard alloy will take any reasonable velocity you can obtain from a .357 or .44 mag IF the bullet lube is good. Some petroleum based lubes can work for low speed stuff but stick to the old NRA foumula Alox/beeswax type lubes for optimum protection.
Pure wheel weight metal typically holds hard antimony crystals in a matrix of soft lead. That can cause leading even tho the bullets feel VERY hard. The addition of some tin is really helpful because it helps the antimony remain in solution during cooling AND it helps the molds to fill out nicely.
I have dispensed with gas checks for handguns, they seem pointless below maybe 1,500 fps. I have found no effective difference between Lee's Tumble Lube/Alox bullets and conventional bullet types if everything else is also properly done.
"Proper" bullet diameter is actually most important with low pressure/low speed loads because they will neither seal the bore on their own nor swell/obturate when fired. But, with full power magnum loads it's difficult to make a bullet alloy too hard for upset and full bore obturation even if the diameter is a few thou smaller than the bore. However, you must understand that any bullet that does not fill the bore OR obturate WILL lead very quickly no matter the alloy or lube!
Contrary to a lot of people's current practice, there is absolutly nothing to be gained by sizing cast bullets larger than actual bore diameter and anything larger than 1 thou over bore size is generally harmful to accuracy.
Accuracy demands a good, undamaged bullet heel. It's VERY easy to cut or shave cast bullets so proper case mouth camfering and expansion/flaring is imperative. Perhaps the very best helper you can have for that is a Lyman M expander die or Redding's copy of it; use it correctly and you cannot damage the bullets during seating.