Keep in mind that a high-level caster typically has ways to sneak his rest periods in if he needs to. The epic-level party I run for typically needs no more than an hour to recover their spells. High-level characters can build their own demiplanes, go to planes with different rates of temporal flow, etc- they're
very good at finding their weak points and coming up with ways around them (at least in my experience). Heck, if you have converted many of the old 2e spells, there's a 2nd level spell from the ol' Tome of Magic that does the trick once per day.
Also, even
one quickened 9th-level spell is enough to totally outshine most other high-level non-casters; for instance, a wizard who can cast both a meteor swarm and a quickened meteor swarm in the same round is pretty durn lethal.
I think the key to checking whether your system is balanced or not is too compare a character's list using the system to a character's spell list who doesn't use metamagic at all; and from my glancing at it, it seems that your system makes for a much more powerful caster at high levels.
At lower levels it seems much better, when you can only use spells up to about 3rd/4th level. Until you have the slots to burn on pumping up your higher-level spells, you come out even or worse. But once those high-level slots start coming, there's no real reason not to burn a few low-level slots to pump them.
The ability to kick out tons of spells per round is a dramatic powerup imo. That's why you can't do it anymore with haste. Even at the cost of a 4th level slot to Quicken them, I think you're underrating the value of an extra spell per round. Especially when you can quicken your highest level spells!
Another thing to consider is that you're capping the number of times per day that the characters can use their metamagic. (If you only have four 1st-level slots, you can only extend four spells.) This may or may not be desirable; that's a matter of taste.
Anyhow, I strongly caution you to keep a close eye on this system if you do use it with a high-level party. You might try making a 17th-level wizard, then pick out two spell lists (one using no metamagic, one using your system to full effect with quickened maximized empowered 9th level spells) and have him fight himself. Betcha I know who wins.