As I know Exxos has ST version, so for DIL 68000 socket.
I don't have experience with such accelerators.
Overclocking is not possible without lot of other modifications. ST's whole design is based on concrete bus speed, so rising it will make video and other things to not work properly.
Simplest solution is to replace CPU to 16 MHz one, and giving 16 MHz to it only when internal operations are performed. When CPU accessing RAM or some port, ROM clock goes back to 8 MHz. Only couple chips needed, but average speed gain is only some 20% .
If you can get Mega STE - it has good and efficient solution with cache. Or get MIST. But look first what Exxos offers.