It depends on whether the speakers or the amplifier is the limiting factor right now. It's kinda hard to describe the difference in sound and swapping in borrowed equipment is usually the best way to tell.
EDIT -- also, you don't want to be running the system at the limits and trying to figure out what kind of distortion you are hearing -- that's how you damage stuff
If the speakers are hitting their limits first, ie they are starting to sound "compressed" and not getting much louder as you turn up the volume, the M80s will make a huge difference.
If the receiver is the limit (sound starts noticeably distorting at high volumes) then you should see a "noticeable" improvement in headroom with M80s but not "night and day".
There are two factors which will make a difference if the receiver is the limit :
Sensitivity -- 91 dB vs. 89 dB anechoic (borderline noticeable)
Impedence -- the M80s are 4 ohm (vs. 8 ohm for M50s) so will, if everyone will pardon the simplification, "suck more power out of the receiver" for a given volume control setting and internal power supply voltage. This should be good for another 2-3 dB, also borderline noticeable on its own
Combined, you should get about a 4-5 dB difference which is roughly "another twist of the volume control". Again, this assumes that the receiver is the limiting factor, not the speakers.
Also, if the receiver is the limit and you are not running a subwoofer now that is another option to consider. Adding a sub and running the M50s/80s in "small" would strip off the lowest (and most power hungry) frequencies, typically letting you play the remaining frequencies at noticeably higher volumes.
EDIT -- but M80s are really cool;)
I would be leaning towards the 80s in your situation, assuming the 50s will fit in their new home. I'm running a 630 with M60s and I don't think the per-channel power difference between HK 430 and 630 is significant.
If you are hitting the limits with your current system then you probably play pretty loud and are going to eventually add a power amp AND upgrade the speakers. Amplifiers seem to get better and cheaper every year, but speaker prices seem to be stable or trending up slightly...
