Lighter flies better. That's why they made the real planes out of aluminum and not steel.

The airfoil and wing shape can make a huge difference in how a model carries additional weight. True, you can make a tree stump fly with enough engine and prop . . . but it will fly like a tree stump. It will be less maneuverable (in extreme cases fun things like trying to pull up from a dive and continuing the same heading but with stalled wings - SMACKO!) As far as I know there is no standard or rule-of-thumb for safe weigh ranges. At some point you'll just have a miserable to fly model that won't be controlable and the problem solves itself. I've seen models fold up in flight when the weight and stresses of over-powering the airframe show themselves. A crowd pleaser, but expensive for the pilot.

In the case of the Revolver I'd say you'd have a good sport/stunt plane with a .50 size engine but more of a pylon racer with a .65 Rossi. Flyable and fast but probably a handful to land. You may need to add tail weight to balance the heavier engine - adding even more wing loading. A 21.5 oz engine without muffler vs., say, a Thunder Tiger Pro-46 at 17 oz with muffler seems like a big penalty in weight.