Of course, the big difference between threads and processes remains the fact that the latter need their personal page tables. There is probably quite a bit of room for optimization of this process on the MMU design level left though. (Why recreate the entire page table set? COW could be applied there as well.)

This is not going to happen overnight, but I'm certain that at some point, the effective overhead of processes over threads will be zero. It is very close already, but not there yet.