Like everyone else has said, the Mak is a blowback design. In order to use hotter ammunition, you'd probably need a locked breech design to keep the gun from battering itself to death in short order.
What this means is that with a locked breech, the recoil forces on the slide of the gun are delayed, generally using more energy to propel the bullet than the slide; whereas a blowback action will use equal force to push the bullet and the slide (for every action there is an equal and opposite reaction). Using hotter ammo might up the bullet speed and power a bit, but the gun -- and the shooter -- would definitely feel it as well. Steel or not, the gun would definitely show the effects of the beating after long.
The Mak is a cool desing, but there are so many 9mm Luger designs out there, why not go for one of those?
Wes