300 ft surrounded by trees I would say it probably won't happen, and I'm the optimistic type[]

As far as decreasing the outlet diameter to increase static thrust I'm not sure. I know there is a sweet spot in thrust tube designs, too big and you loose efficiency or too small and you loose efficiency. I don't know any of the equations to justify these statements and not sure how much +and - you can do to increase thrust. But I would think that maybe making the tube a little "larger" may help increase static thrust and I may be totally off, but I know that the larger diameter byron fan units created more thrust than the dynamax, and the dynamax had more top end speed, but I know the actual design of the fan has a lot to do with it too... So I'm really not sure on that part

Only other thing I can suggest is trying to take off on only half a tank of fuel and cut your flight time in half, and maybe try shaving a few ounces off by putting in a smaller reciever pack.

Try that but be careful, don't be in so big a rush to sell it that you do get it off of the grass only to stall it a few seconds after take-off. And don't get me wrong, I'm not trying to tell you what to do I have just done things because I felt it "had to be done" and ended up regretting it.