It's different with OMS and Freemidi.
With Motu gear speed settings in Freemidi tell Freemidi to comply but not the interface. Likewise in OMS. One has to physically through menus set the interface speed independently if that option is there. The two have to agree.
In the case of the Studio 5, it's like you said, as there is no manual switch on the interface itself as with the Studio 4. I wonder if the OMS menus are actually regulating both ends of the Studio 5's communication. But as you refer to changing the MTP's speed, that only changes OMS's end. You still have to physically change the MTP from the MTP itself in the global hardware settings.
Very different...Motu interfaces that I've had cannot change speed modes remotely. So your XT is like my older MidiExpress. There is no way to choose fast mode on the XT itself. Then you have to set Freemidi to 1X, otherwise major communication problems, if any communication at all... Technically speaking, I've heard it said that, actually, the Studio 5 is always in fast mode.