In my car, I have an AnyTone AT-5888UVIII (nominally 50 watts) feeding a Metropolitan tri-band (144/220/440) antenna. I've measured the output on the radio at 5, 12, 28, and 40 watts on UHF. To the GMRS repeater at my house, 5 watts and 12 doesn't seem to matter, but 28 gets me 2-3 more miles in some locations. I don't recall finding a difference between 28 and 40 in most locations; however, when I dip down into the river valley, only 40 watts will get me there. That's my very unscientific study. I have a different radio (TYT TH-9800D) in the truck and an SG7900 "Super Gainer" antenna. I haven't measured the output of the radio, but it's likely about the same. With that antenna, it seems to be pretty much "all or nothing". I do get more farz than with the little Metropolitan antenna, though (unsurprisingly).
As noted above, I did find one situation where the difference between 5 and 10 watts was able to overcome the attenuation of my walls using a really suboptimal antenna. Always remember to never say always or never.