What is more effective than ASIC mining? Considering that mining progressed from CPU, to GPU, to FPGA, and now ASIC mining... have we finally hit the end of the road for technological advancement?

What future optimizations can occur after ASICs are widespread?
Well, the ASICs machines would probably be more and more refined over the years (like processors), they can get bigger and so forth. So that will probably be the progress for a long while.

Asides that, we have the quantum computers that might be the next big leap forward for all fields of computer science, but for now they can only do really small calculations (3x5=15... on 5 atoms, or something like that). However, there is no telling if and when quantum computers might become viable enough to do anything Bitcoin-related with them, or even whether they might be used to destroy Bitcoin's algorithms altogether.