Arisu suggested that if the code was optimized using a compiler, and just focusing on this single optimization Arisu did, the benefit in terms of performance would probably be over 100%. Now if doing other optimizations as well with a compiler, the performance increase would be even higher.
Sure, inlining and simple code reduction alone can have profound results on generated code, and there's a lot more optimizations can do to make code run faster. Personally, I couldn't imagine releasing any of my own stuff without applying optimizations (unless it was for debug/testing purposes), because I couldn't stand knowing my code is running slower than I know it could be.
A lot of the code I release is of my own making, though, and it's entirely CPU-driven. But a game like Skyrim also throws work to the video hardware and 3rd party code, like DirectX and the Havok stuff. Skyrim being built unoptimized wouldn't affect the performance of the drivers and hardware, or the pre-made 3rd party code, but it would be a bottleneck (eg, the fastest GPU in the world won't do diddly squat if the CPU can't work fast enough to tell the GPU what to do). Optimiziations would definitely improve overall efficiency by having the CPU do less work, but how much of a noticeable performance increase depends on how much of it is being bottlenecked by the unoptimized code, and that's very hard to guess without seeing the source code and proper profiling and timing metrics. Arisu's patch shows there are definite gains to be made, though, particularly on lower-end CPUs.
My amature guess is that with all these performance optimizations done using a compiler by Bethesda, the performance increase would be so high that Bethesda could actually add in a bunch of not-outdated graphical features as well, like parallax occlusion mapping to make textures not look 2D flat, and more updated lighting system (global illumination perhaps?) and updated shaders and especially a better LOD system (the LOD system they have now is a complete joke for 2011; it's just as bad as Oblivion's except that we got a few 2D distant buildings here and there, but the pop-up apart from that is still just as bad).
The shaders are entirely GPU driven. The shaders are passed to the drivers, which do its own processing on them before sending them to the hardware, and the hardware runs it. The lack of optimizations made when building the executable wouldn't affect the drivers or GPU performance, it would just affect the performance of Bethesda's own code that tells the drivers what shaders to use.