It depends on your terrain, among other factors.
I live in a valley. Usually 5 or 10 watts is enough to hit any of the
local repeaters. Increasing the power beyond that doesn't add
many, because they are more limited by terrain and/or multipath
(which is relatively independent of power). Sure, I can hear more
distant repeaters, and often kerchunk them when I raise my power,
but that doesn't mean I have a clean signal into them.
Not that higher power doesn't help sometimes, but other times
repositioning the antenna by a couple feet makes more of a
difference.
And, of course, if you can't hear the distant repeater well, then
higher power isn't going to improve that.
Now, other than receive ability, the situation may be different
over fairly flat ground where you don't have multipath.
But otherwise, if you already have your antenna up as high as
possible, and are using reasonably low-loss coax for the length
and frequency, then a beam antenna of some sort might make
the most worthwhile improvement. Or switch to SSB / CW or
a digital mode like FT8, which work much better with weak
signals than FM does.