we will try to include the large file support in 4.0b3 release.
It has to be either enabled or disabled at compile time, not possible to have both being controlled at runtime.
Anyway, the performance difference should be negligible in real world usage.
What about having multiple versions of your standard/enterprise servers, both with and without largefile support? That way, if a user needs it, then they can have it, but if they don't, they won't suffer the extra overhead.