Considering Flight Simulator is heavily bound by CPU and DirectX 12 has a lot more functionality to handle parallel threads....yeah, I expect a good deal of improvement. I think we will see this improvement on Xbox consoles carry over to PC when they get around to it. Not every game has the CPU demands MSFS does and so the upgrade to DX12U will be welcome. Regardless, when that happens, only then will we be able to truly compare.
The developers themselves have said that DirectX 12 won't improve things much if at all. It may instead enable them to do things like ray tracing and other DX12-only effects.
The optimization that people expected to come with DX12 is already coming tomorrow without any need for DX12.