Well in general repeaters are put in places of advantage. High hills, tall masts, higher gain antennas. With UHF power isn't really everything, height is.
If you are going to use a unity gain antenna 10' off the ground with the repeater then it will act more or less like another HT. But if you can put it on a 50 foot hill with a 20' mast and a 7dBi gain antenna then it will perform much better than the HT ever could. More power to punch through some kinds of obstacles such as a line of trees, some buildings is always helpful if they are there. But if you have a mountain in the way no amount of power is going to help you.