Will the move to CoreCLR give any speed ups in practice if the release build is complied with IL2CPP anyway? On all the games that I've worked on, IL2CPP is one of the first things that we've enabled, and the performance difference between the editor and release version is very noticeable.

Editor is slower than Mono release builds. You'll need to compare Mono release vs. IL2CPP release to see the actual difference.