The MU2000EX is not at all an overkill synth. I love mine, and its great for many GM based games. But quite often, the so called GM based game is really meant for an SC-55. If you try to play those games on any other synth, you will hear anything form bad/wrong instruments to the melody being totally out of wack. One of the great features of the MU1000 and MU2000 is the EX patch. It upgrades the units Yamaha 300B mode (which is like a lite version of the SC-55 soundmap but kinda poor), to an official Roland SC-55 soundmap. It still uses Yamaha samples of course, but it still sounds very good and you get the Yamaha drums which are quite nice. So you can just switch to SC-55 mode if you run into a "GM" game that doesn't like Yamaha's native GX mode.
I would always recommend getting a real SC-55 first though as they have such a great sound signature that just cant be copied 100%, and the going price of the MU1000/2000 units is still pretty high. I feel that I got a good deal on mine at about $200.00 but thats not great. Plus I had to pay another $65.00 for shipping from Japan. Compare that to the SC-55 I listed a couple of pages back, and I only paid $5.50 and got everything under the sun with it. But if you have other midi game rigs like the X68000, Atari ST's, and even the Amiga line (though not many games for the Amiga used midi), then its a great value in that price range.