Earlier quoted context omitted.
Small clarification. It is faster than the old Mono they ship. Their Mono is at best Mono 2.10, about three years old, without any of our optimization work for three years, very notably, a lot of the LLVM support, or the new garbage collector. That said, the il2cpp looks like a very promising an interesting project, so kudos for that!
Yes, I think those benchmarks are based on the Mono they use, which is indeed old. I would be very curious to see benchmarks compared to trunk Mono - should be interesting. My information may be out of date, but typically people tell me that Mono tends to not match Microsoft's .NET implementation in speed, and in turn, .NET tends to not match native C++ either. So I would still guess il2cpp can win here. Or are there…
C# is ment to be a safe language, unlike C++ which has, among other things, the notion of undefined behaviour and manual memory management. This is what makes C# initially slower than C++. You might be able to compensate with a JIT, but safety still costs. Not to mention, the Unity guys will still probably have to implement their own GC and spend years on optimisations.
Also, several games are using dynamic loading for things like plugins and mods, as far as I know, il2cpp will force them to change how this is implemented.
From my standpoint. The cheapest option, and best option for everyone really, is to continue using (an up-to-date) Mono and instead allow for writing games in C++ as well.