Underpowered? No..... Using base spec's for the old fogey wing area, and adding 4 oz for the additional weight, he's well within normal limits. Right about in the middle actually. Actually adding 8 oz of additional weight only brings the flight time down to 14 minutes (versus 16)
Nice graph, but unfortunately it can't increase the motor's power
Does your calculation model give the motors output rpm? If so, I can put it in the Phoenix simulator to see how the plane flies.
I normally reckon on about 100w for each 1 pound of flying weight for a reasonably flyable plane. You can use as low as something like 50w per pound for an under-powered trainer plane. I think that it's always better to have more thrust than you need for easy take-offs
Say OP was going to use a trainer plane like yours. He needs to add 100g for the load and something like 50g for the release mechanism, so the total weight would be 750g =1.65 pound. Therefore, 165W would be OK, but I'd personally want at least 200W. It just depends what he wants in the way that it flies.