The masteron effects would simply diminish and become less pronounced.
I've ran Mast from 400-600 and I think 500 is a great dosage (Not too much, not too little)
Anything less than 400mg wouldn't give you the benefits or full potential of the drug.
Mast is very mild in terms of sides IME. Practically side free.
With that being said, Tren is very effective at lower dosages and most overuse/abuse the drug.