My understanding is that the boost Gears is supposed to offer is actually built into Firefox 3.X already, making Gears unnecessary:
Sorry, but no, you understood wrong.
Firefox 3.0 has functionality from HTML 5 that is similar to Gears, but it is not Gears and it is not compatible with Gears. WordPress does not support this functionality yet, because with Firefox 3.0, there is actually no benefit to using it, it doesn't speed up anything.
Firefox 3.1 will have improvements that allow use of those features to cause actual speedups, but until that happens, WordPress does not support it.
Also, Gears adds a heck of a lot of extra functionality that WordPress is looking into using for enhancements as well. None of which is in HTML 5 or Firefox. All around, Gears is a heck of a good development platform, IMO.
I ran Firefox with and without Gears using 2.6.5 and recorded no discernible difference in response times.
I'd have to disagree with you there, the difference is immediately noticeable to me, especially on the edit posts screens, even on very fast connections.