That could go either way. Apple does update machine firmware when it rolls out these major upgrades. Some of the smaller "all in one" updates will, as well. Especially if there is a pressing need. I don't think that they would just shut the door on a convenient upgrade of older hardware. There's not any financially motivating reason. In fact, keeping older hardware in the market has been one of Apple's strengths for decades. You only upgrade when you want to. Look how long they supported Carbon executables after moving on to Cocoa. From a purely technical standpoint, if you haven't had a problem yet, it is PROBABLY not a factor now. Also, I can't really see why the effect would be to hose your mouse. There are much more memory dependent Apps.
I know that you are looking high and low for a solution. Despite my banter with H, I'm not going anywhere while people are still having issues. Maybe I can help. Maybe not. My Solution post does actually work for many people, including myself. The remaining issues are concurrent problems, like Len's and the headphones, and Microsoft BT mouse users.
I FLAT OUT CANNOT BELIEVE I'M ABOUT TO SUGGEST THIS, BUT HERE GOES! "...though I suffer the slings and arrows..." Ducking now... INCOMING!!!!!!! you could try installing the microsoft intellipoint for mac stuff, to see if you get any improvement. Okay, I'm still here. The color is coming back into my vision. Heart rate is below 140 now.
My research on the Microsoft users falls into two categories. Gamers, and everyone else. Often, gamers will have issues of "sniper lag," my term, where the mouse has entered idle mode from a lack of activity and the first motion of a sniper moving to correct aim for a snap shot will be missed as the mouse transitions from idle to active. On the other hand, there have been problems with IntelliMouse mice dropping the BT connection in "Dormant" mode. "Dormant" mode seems to be the culpret here. Generally, the mouse will enter it Dormant mode after 10 minutes of inactivitiy, but many gains in battery life are a result of dropping to Dormant mode after shorter periods of inactivity. In an ideal world, the Dormant mode IntelliMouse would reduce only it's sample rate on the optics and not fiddle with the BT connection. In the real world, the BT connection is what eats power, so reducing radio power, sending "keep alive" packets less frequently, and other tricks can have a fairly large impact on battery life. Normally, such tactics will not negatively impact the typical user.
I have begun to suspect that there may be a problem cropping up as Apple has moved to the BT 4 specification. BT 4.0 is the low-power, energy saving spec. It's possible that a very strict adherance to the spec by tweaking the settings of older "pre-4.0" hardware may be resulting in dropped connections. Whether this is due to the Microsoft implementation of the BT spec, or Apple's I cannot say. In any event, that's my thinking on your mouse problem. I'm willing to keep trying as long as you are.
Let me know if you try the IntelliPoint software, and whether it helps.
Cheers!
Cal