Are there any asio performance improvements upgrading to Windows 10 from Windows 8.1 using Cubase 8.5?
Unlikely.
Have seen one user on gearslutz claiming greatly improved performance coming from 7 to 10 but every other report I see and my own experience is that performance is pretty much the same.
I agree with Grim. Likely there are few differences between W8.1 and W10 in regards to Cubase 8.5.
I say this because there has been relatively little time since W10 was released, and there was an issue with C8 that took a while to fix because testing on Steinberg’s part was only started after W10 official release. Nothing wrong with that at all, but that seems to speak for itself in terms of your question.
This could also be made as an argument for not delaying an upgrade to W10, since it’s probable Steinberg will have commenced adoption of the future of Windows. 
I believe there are threading related changes in W10, how that affects Cubase I have no idea. It’s likely in some software combinations ram handling may be different.
Also there is the RAM compression now in W10, which compresses less used process in memory which might have a slight effect…
MIDI is improved in Win10, timing being the only benefit for Cubase or any program using midi, right now. Hopefully they will take advantage of the new midi API in the future.