an 030@50 with good ram will tick over at about 10mips.
cooking on gas for basic amiga use, 3D floppy games and the like.
an 040@25 having to struggle to get to mainboard ram on an A4000 will get about 19mips
a proper upgrade. the first thing i wanted for my A3000 if i got nothing else was a cheap A3640 card. very sufficiently nippy to make web browsing tolerable, and rendering a workable reality. you can feel the difference in day to day use.
an 040@33 with local 32bit ram should get around 30mips.
starting to get into demoland terratory. more cpu intensive tasks become "just a thing"
i had a warpEngine040/28 overclocked to 40mhz with a big heatsink on it. it rocked.
later i had a Cyberstorm2/040@40Mhz i couldn't afford an 060, but it still enabled lan quake1 gaming with a graphics card.
the more CPU you have, the less running things like IP stacks and USB stacks in the background are a bother. 256 colour icons become a reality, mui becomes a "whatever" rather than a system drain or resource hog.
but some stuff like microprose F1 GP is artificially limited to say 12fps, so you have a consistant game expirience across all platforms (tweekable with F1 GP Ed)
other stuff like Populous and Syndicate run their game engine clocks off a loop from an assumed CPU speed of 7Mhz - result is anything above an 030 it becomes entertainingly unplayable.
other old polygon stuff like Indie 500 eats cpu cycles for breakfast and takes all you can throw at it with a big happy smile on its face.
pretty much all if the above is just my experience and anecdotal if you will, but i hope it was usefull in some way.