This can be done quite easily now by modifying the spell damage for the unit when it is cast using the new natives that were added as part of making Reforged.
The limit is with faster attack speed. The range is -80% to +400%. Using triggers one can modify any unit attack speed up to 50 attacks per second using the new natives that were added as part of making Reforged.
StarCraft II is limited to at most 16 attacks per second.
Allowing that to be changed would likely be non-trivial. I can imagine the current hero attributes are hard-baked into native types.