It sounds good, but I don't see it being that helpful. Ubers are helpful because of their ability to push forward, allowing an invincible teammate to push past the line of battle and take out sentries, Demomen, etc. Without that vantage point the crits seem nice, but ultimately unnecessary. Heavy is obviously the best choice for the uber, 100% crits for 10 seconds would make him a wrecking ball. Ideally you couple that with an invincible uber that takes the fire for your crit uber to mop the floor while hell is breaking loose.