I would like to modify a GP Super Sportster 60 to a V-tail configuration . I figure the top angle between stabs should be 115° . Not sure how much to increase the area of the stabs/ruddervators .Has anyone tried this ?

110 degrees is pretty much the sweet spot. I have to ask why you are wanting to do this? You would be seriously limiting the airplanes performance.

I recently did the same thing with an Advance 40 that had half the stab broken off. Took the two halves and made a V tail, about 115 degrees. project went quick and easy. Did not change size of elevators and they worked fine, just make sure to check and readjust CG as needed. Very good flying plane.

I have other planes with really good aerobatic performance.......zero with a v-tail.....I would like to explore the envelope of a v-tail .

Looks great . How did you build the tail section ?

Actually very easy, took the two halves of the broken stab and cut them the same lenght. Got the correct angle (anything between 110 and 120 is good), glue the two together, then trimmed a piece of wood at the correct angle to glue in the middle. Just for a little more reinforcement, put a small piece of plywood inside fuse and ran a small screw thru the center stab assemble after glueing all on. Had to also shorten elevators a little to fit.

Although I used to race pylon with V-tail planes, I kind of did this as an experiment to see if there was any big difference in flying. I expected some tail wag and less sensitive elevator , but plane flew excellent, almost seemed to be more stable. Had to add a little tail weight, I guess because I lost the fin and rudder.

Ok I doing it .

You may have to increase the size of the V=tail surfaces to about 130% of the original elevator/stab area to get the same pitch control that you had with the original stab/elevator surface area.

Not really disagreeing with you, that's kind of what I thought too, and i'm sure it can vary with different planes. But in my case, I was too lazy to cut new parts, took the old broken stab and actually cut them down to make them even (elevators too), and pitch control is very good.