Hi.
It's impossible to answer because the schematic doesn't tell wich transistors you use. That is cruicial info to determine the mosfets resistance between source and drain. That's what you're asking for, right? Put in more mosfets in paralell to reduce the total resistance.
What about using bjt's instead? Probably the total efficiency will decrease (at low load), but you may be able to take out more effect. Also buying a larger transformer will help, as the iron in small transformers will saturate easier.