Only the MU1000 and MU2000 could be upgraded with the EX firmware. It was the EX firmware that allowed them to offer real GS support. More or less it allows the modules to use the same sound map as the SC-55, but with Yamaha MU series samples. The GS portion of the firmware is licensed from Roland so its official GS. Just remember that uses Yamaha sound samples as well as the MU's onboard effects so it still sounds different than a real SC-55. On the MU128, MU500, MU100, and MU2000 there is a mode called TG300 (not to be confused with another Yamaha midi module that was called a TG-300), and this offered a type of GS mode but could not be called GS as it was not licensed from Roland. Its also is not 100% sound map of the SC-55, but still sounds pretty good. The EX update is a worthwhile improvement if you want GS support though.
Also, not all MU1000's and MU2000's have the EX firmware installed. It is not as simple as running a little batch file and saying "yay, I now have EX". I have an MU2000 that I patched to be an MU2000EX. I did this over a year ago and don't remember all the steps, but do remember that I was sweating a bit because its REAL easy to brick the unit. Period. It can only be done on certain OS's using multiple programs and multiple updates done in phases as there are several EEPROMS that get flashed in a certain order. If you screw up, you keep the unit for spare parts and spend a few hundred more buying another module (plus shipping). The flash programs, and files can only be pulled down from a Japanese IP (I had to be a bit creative there 😄) from Yamaha's Japanese FTP site. Not simple to navigate if you don't read Japanese. The instructions are of course only in Japanese, and the units instructions will only be in Japanese. That's because the MU1000 and MU2000 were never released outside of Japan. It also takes about 10 minutes per patch so expect at least a half hour of sitting and staring at the unit and your screen. You have to wait it out as the thing will time out and mess up the install if you don't look for the right screen to show up.
Never have I seen such a complex thing to do a firmware update on. But well worth it. Several hundred more instruments for XG (with some fantastic Yamaha string, horn and bass samples), real GS support, and some improved effects. It also allows the use of 64MB and 128MB SmartMedia cards, but eh. With USB on the MU2000 I could care less about the long dead SmartMedia card.
If you want a Yamaha XG module, then you go small with something like the MU50 that can buy on ebay for under a hundred from time to time. Or you go big and get the MU2000(EX) since it offers everything under the sun and for not that much more than all the other post MU50 modules. So about $100 for the MU50 or about $300 for the MU2000. The MU1000, MU500 and MU128 average $200.00 plus shipping and lack TOSlink, USB, and lack lots of other bits that make the extra $50 worth pumping into the MU2000. Shipping from Japan is a bit high, but there really is no other option for these modules.
Then you can add PLG100 and PLG150 cards to expand the unit. I currently have a PLG150-DR in mine. The past year the price on these cards have started to shoot back up. Last year seems to have been their low point. You could buy many of them for between $45.00 to $80.00 if you looked around. Now they are back to their original selling price in the $150 to $300 range. With shipping you can max out an MU2000 with over $1500.00, so keep it simple. Better to go with a modern Motif instead if you are spending that kind of bread as the Motif's can do XG.
So if you really want to hear Final Fantasy VII in all its glory, play and make XG music and hear it at its best with a TOSlink, then its money well spent. But if you are just looking for basic midi support, then you can do it for far less money. Also when buying from a Japanese seller on ebay, they will try their best to answer your questions but more often than not they might not really understand you. Example "Does your MU2000 have the EX firmware update already installed?" The answer may be yes and in reality it wont be. Many of the sellers see printed info about the unit and have no idea it has to be upgraded and the assume it has it. Or they have no idea about what it is and don't really understand your question. Also they may have a dozen of them to sell and use stock photos with the MU2000EX production unit that came from the factory with that lable and firmware already installed. Yamaha only did this the last six months of production so they are rare units. More often than not, they will need to be upgraded by hand.
When it comes to shipping you may have to wait a while or you may be surprised at how fast it gets there. I paid about $200.00 total with shipping and it was at my front door in Atlanta Georgia within two days. I've never before or since got anything from Japan in 48 hours. That really impressed the hell out of me.