When it comes to games, UWP means only "C++ and DirectX 12", so from a performance standpoint is perfectly fine.
Unless you are talking about solely Xbox One development, there is no such restriction.
Its entirely possible to write a game in Silverlight using DX11 and release it as a UWA.
Also sharing 95+% of the code base between the pc and xbox builds is a huge win in maintenance and bug fixing.
Not really, because they won't share 95% of the codebase, and if you expect to sell copies on the PC you don't release games with as many restrictions as possible on them so you will end up with one X1 branch, that has to go through cert to patch, one UWA branch that has to go through cert to patch and one traditional PC branch patched as bugs are found.
In essence you've just doubled the workload on PC development and maintenance for... I dunno, a value add to your X1 customers?
e:
I mean, you would really need to define what you mean by "95% of the codebase" because a PS4 game and an Xbox One game on modern middleware share "95% of the codesbase".
The "95%" is shit only a handful of people on the planet worry about.
The meaningful problems are in the code that ONLY works on one specific platform, and that scenario does not change at all until all PCs have unified memory pools / GCGPUs / many weak cores instead of fewer stronger cores etc etc etc
Or in other words when all Pcs are exactly the same as Xbox Ones, which is never.