Yeah SLI is dead, has been for a while really but that is my point - there is no worthwhile “upgrade” from what he’s specced, regardless how much money you are willing to throw at it.
DX12 doesn’t magically use more cores, thats a common misconception. Port the engine over to DX12 without doing anything else and it isn’t an automatic performance win and all of a sudden your game can effectively utilise as many cores as you can throw at it. Equally there is nothing inherent about DX11 that prevents efficient use of multiple cores - case in point star citizen is still DX11 (Vulcan work is ongoing) but has by far the most efficient use of up to 32 cores I have seen in any video game due to the way jobs are dispatched across threads.
Also MSFS does already use multiple cores, the problem is that it bottlenecks on the mainthread so even if you add more cores it doesn’t matter, that mainthread is the one dictating your max CPU frame rate. They could effectively dispatch to more cores than they do now, but without improving the mainthread bottleneck it would have zero impact on frame rates. The good news is they apparently continue to methodically try to move things off the mainthread as time goes on (it is difficult work to ensure everything remains threadsafe) so things will improve as time goes on.
There are definitely improvements to multi threading DX12 can bring, not least in how it handles draw calls, but the really interesting stuff for DX12 is really more on the GPU side.
The last dev video mentioned some significant performance wins (such as 4x speed up on trees) during ongoing work in optimising for Xbox and DX12 so it is encouraging that there will be ongoing improvements. I’m sure as a first party their implementation of DX12 will be excellent.