Gamers or enthusiasts in general serious about performance will try to keep their os in pretty good condition and recouses used to a minimum anyway so the difference in game mode probably wont be a great deal. Id imagine it will be a boost for grub users.
FPS and Game Mode: This CAN and WILL improve FPS in some circumstances but it's all about improving how the CPU works. If the CPU isn't a bottleneck already for your game then there is NOTHING they can do to improve FPS.
But... they could force CORE#0 to run the game thread ONLY (nothing else) and disable hyperthreading (CORE#1). Sometimes a game has a CPU bottleneck because the main game thread is running on a core that is in turn running OTHER game threads or Windows applications.
So for performance with GAME MODE it's all about looking at how the CPU is used and asking what can be done to fix that.
OTHER things they may do is look at CPU architecture such as RYZEN such as CCX thread jumping though not sure if that is something Game Mode would address or whether the game developer (or Windows thread management) would be involved in. Perhaps ALL of the above for optimal usage.
WEAKER CPU's would see the most benefit because they are most likely to have a CPU bottleneck. For example, an FX-4300 for a particular game you might want to use CORE0 only for the main game thread and NOT use CORE1 (if that's the other part of the module) for maximum usage.
For the above that assumes that the other module (CORE2 and CORE3) can handle the rest of the game threads, or at least do so BETTER than the previous method.
Not me. Got it clear. Core lockouts. Going to be interesting to see exactly if that idea works, and it'll work best on cpus that have not only the room (i7's, Xeons, R5's and R7's) but on quad core pc's it might actually be a detriment. Win10 especially is rather thread invasive, there's always crap goin on in the background, which is using threads. When you take a game like BF1 which uses as many threads as it can get its paws on, my guess that locking up dedicated threads (say 3) would mean that the game would actually loose access to 1 thread, as that's now relegated to Windows etc by default. So instead of running with access to all 4 threads, now it's stuck with a max of 3. Of course the biggest winner in all this would have to be the FX 6300 and the R5's. With 6 threads possible, dedicating 1 to Windows and leaving 5 dedicated to game mode would sure speed things up since Windows background tasks no longer get any sort of multiple thread priorities.