Billy,
See, ask a simple question and get a dissertation.
Well, considering the lift potential of the wing (640 square inches and an airfoil similar to a TP) and the size of the flaps, too light and it will be uncontrollable in corners and twitchy as heck. When the lift potential of the wing far exceeds the payload it's asked to carry, it can make for some interesting flying. Ask Ted sometime about his experience with the "Too Light Tucker".
I've only had this happen with a plane I've built once (it's usually the other way around; a tubby plane with not enough lift potential to carry the load). A 9 to 1 AR plane with a 620 square inch wing. The plane weighed 41 oz with a very, very long tail moment and TVC of up around .62. It was entirely unflyable until ballast was added. At about 55oz it became marginally flyable. At 64oz is was a pretty good plane sans the usual problems with high AR planes. Had the most consistent, killer turn of any plane I've ever owned but that didn't happen until I got it over 60oz. Payload has to be balanced with lift potential. Depending on design, the window is usually pretty wide and you're better off being on the light end of the envelope. Some designs, like the high AR job, the window in much narrower and due to other design factors, makes it intersting to fly if your out of the window at either end. The further out you are, the more, ah, challenging it gets.
It's a matter of design. If this plane is sub 50oz, I would need to severely reduce either the amount of flap area or mess with the control linkage to substantially reduce the control throw (probably near the 2 to 1 mark). It is designed to come in around 55-58oz and would probably be ok up to as much as 62oz. At, say, 57oz, it should be a real killer. At under 50oz, it would be quite a challenge to fly consistenly. We'll see. I'm normally something of a lead foot with a spray gun so I'm not expecting it to be under 50. My best guess right now, based on what I have so far, is 55oz.
The current design I'm flying (on which the new plane is based) was originally built a bit under 49oz. It was a marginal plane to fly, especially in the wind. Had a very inconsistent corner. Really hard not to bounce and just really unpredictable. It flew OK, more or less, in the calm, kinda, but in the wind it was a pretty wild trip. But a change to 15% smaller flaps with a few degrees less throw and the addition of about 5oz in ballast (mostly for trim reasons) made it a pretty decent flier. The change from an OS40VF to a RO-Jett 65 didn't hurt in dragging through some tough spots, though I had to add about 3/4oz of lead to the nose to compensate for the reduced weight of the engine and keep the CG consistent. Current weight of the plane is 54.5oz (on 625 squares) and it flies rather well.
Like anything else, weight (or I would say, rather, payload) is another design factor. I hear guys say, "lighter is always better". I would say, lighter is usually better, depending on other design factors.