PA28: Difference in constant speed prop vs fixed pitch

I have a PA28-180 with a fixed 60inch pitch prop. I have always
wondered how much faster it would be if it had a constant speed prop.

I'm hoping someone with a PA28-235 (which has a constant speed prop)
could look up their speed for a reduced power setting, for example 57%
which would correspond to 135hp (which is 75% of my 180hp).

I know it isn't a 1:1 correlation because the 235 will be heavier, but
it is basically the same airframe, so it might be an interesting
comparison.

As a side note, My PA28-180 has an O-360-A4A, which has a solid
crankshaft, so the only CS option would be via an electric MT
propeller (for which they do have an STC). I'm told this is a $10k
prop, so you'd have to see a heck of a performance gain to make it
worthwhile!!!