Have come to that conclusion for most of my mid-range cast bullet shooting...will size the bullet to whatever the diameter just ahead of the chamber. Figure the bullets are going to become that diameter when fired at this pressure level so may as well start out that size (so long as the case has enough room for a clean release).
Don't be quick to blame leading on velcity. Leading that starts at the breech and works forward is most often casued by too small a bullet, too hard a bullet for the pressure. Velocity leading (or lube run-out) will start at the muzzle and work back (if speed is the cause, then it's going to happen where the bullet is going the fastest).