I became a Mini Route "convert" a few years back. HDD's are comparatively cheap nowadays so the extra storage space isn't an issue. It also means that you can customise stock in each Mini Route. Errors only affect ONE ROUTE if you introduce new stuff with errors, so it's easier to find and fix. ALL utilities can be "pointed at" each mini route and work as per a normal MSTS installation. Mini Routes are easily loaded into OR should you want to use that too.
It would seem that I'm in a minority using both OR and MSTS but it's for the reasons that you have. A lot of UK stock's physics don't work properly in OR as the UK scene spent a lot of time "bodging" MSTS physics files to make stock run more realistically. Unfortunately although this gives excellent performance in MSTS it's generally hopeless in OR.
I also have an AMD based machine but a couple of years back bought an Intel machine with NVidea graphics just to use as a SIM machine. Are you sure that the AMD graphics cards are not the issue? Whole heap of posts on that here.
Dorset - near The Swanage Railway.