MTBs don't really tend to have armour. At least not enough to withstand 40mm rounds. A high velocity 20mm should be more than enough - the problem will be getting a high rate of fire and the weight down.
Exactly: if your goal is to kill MTBs with fire from the air, regular HMGs or 20mm cannon will work fine (especially considering how vulnerable to fire most of them are). The FAMs current Fw-187s, with their twin 25mms, should do the job nicely..