I have it. No issues here with frame rates affecting running the route, even with a long consist, but would strongly recommend a higher-end computer setup, especially when on the frame rate intensive NY-NH route. I normally get between 30-60 FPS on most routes, with NY-NH coming in at the low end of that range. I also have run the M8 on the NEC Philly to NY. I run the sim with all settings maxed.
My specs: Windows 8.1 64 bit, Intel Quad 4770 processor, 24gb RAM, AMD R9-270 graphics card, 225gb SSD, plus 2tb data hard drive. 23" hi-def. Dell Touch Monitor @ 1920x1080.
I can't speak to how authentic DTG's M8 is, but IMO it has a beautiful, sharp, detailed look. It's one of the better locos I've got, with respect to appearance. No issues with how it runs, but again I'm no expert on how authentically it's set up regarding physics, etc. Other folks here would know better than I.
I do have a problem running it in Expert Mode with what purports to be the authentic signaling system on NY-NH. But that's more due to operator inexperience than the loco itself, I think. So most of the time I run it in Simple Controls. Gives me more of a chance to get out of the cab and get a good look at the details of the route. Please don't tell anyone I do this!

Screen shots attached, if I did it right.