I own the FMC-06 Mutec flash, I bought it instead of the Yamaha flash for my MoXF. It is the 1GB size. It's produced in Germany, and experiences are very good, you can see many reviews for the Mutec board at thomann.de.
For the new 2 GB version however, I'm still cautious. In a German forum we're currently contacting Mutec for more precise information. But I'm sure they tested it thoroughly for the Tyros 5.

German stores sell the FMC-06 Mutec board together with The MoXF, they definitely wouldn't do this if it voided warranty.

For the MoXF, you have to consider that even if the flash storage capacity is not yet fully utilized, there is a maximum number of waveforms and individual samples the MoXF can handle, so you might not be able to use the 2 GB to the full extent depending on the structure of the waveforms. I don't know the details of the Tyros in this respect.