...
I'm not sure if the GMW thing is always so overpowered. At level 9, you could use it to make a weapon +3. Now this would be a 3rd level spell for most who can cast it, which you don't have *that* many of. It's a pretty important spell slot to you.
Now if you rule that you can never surpass the total (equivalent) enhancement bonus, this is a pretty bad spell (depending on the weapon you have it's either plain bad or just rather weak). Let me explain.
It's reasonable to expect that any (combat-oriented) lvl 9 character has a +2 weapon (unless you're one of those DMs who thinks player should make do with wooden clubs until they reach epic levels).
Suppose you have a +1 <+1 equivalent quality> weapon (+2 total), so now you're getting +1 attack/dmg for 9 hours. Not bad, but there's easier ways to accomplish that (Bull's Strength comes to mind, though that doesn't always stack). You *can* now use it to beat a DR 2 opponent, but those usually can be dealt with anyway. When you meet that DR 20/+3 monster however, you've just wasted a spell to turn your weapon from useless to even more useless.
Suppose you have a +2 weapon, now you're still getting that +1 attack/dmg for 9 hours. Same comments (your bonus isn't really that spectacular), but now you can actually hit the DR 3 monster.
I don't know, but it seems to me that this just encourages players to only buy +x weapons in case they run into that DR 3 monster (which will be high CR, so that's when they need their hitting power the most), which is terribly boring. Anything but straight +x weapons will be useless in the big, important fights so often it's not even funny.
Now with GMW, you're switching the situation around a little: pure +x weapons aren't that attractive anymore. I don't think that's such a bad thing however, I even suspect the designers may have made GMW to encourage people to take non straight +x qualities for their weapons. After all, the way DR works means that otherwise nobody would be seen with anything but a +<maximum they can afford> weapon.
Now as with all things, you have to use moderation when considering this semi-official endorsement of non +x weapons in the form of the GMW spell: if you overdo it (+1 keen flaming shock longswords), GMW isn't terribly balanced anymore.
To me, the main problem seems to be the fact that it's always a lvl 3 spell, which makes it too cheap to use at high level. It scales too well *and* you can make wands of it. So perhaps the original spell should be limited to +3 weapons, with a new spell available at spell lvl 6 that allows you to go all the way to +5.
In fact, I think that last one sounds really good. You couldn't make wands of it either, which is nice.