Due to the relatively small cant angle I believe yaw stability and control should be increased by a fair amount. Also there should be a small increase in pitch authority associated with having the tails canted. This increase in pitch may help somewhat with STOL operations when you need to takeoff more quickly.
If you look closely they didn't even reach a consensus of just what one is supposed to look like. Antennas different between prototypes. The nose cones are different. One has a reduced signature intake whereas the other uses the boxy intake of the original Saegheh prototype. Some of the prototypes have no guns, at least one has a single gun. Even the exterior lighting isn't consistent on one to the other.
Seriously, twin tails are most often needed on Mach 2 capable jets as the directional stability drops off significantly at those speeds. I doubt this jet is any faster than an F-5 and it did fine with one tail
<b>"It's time to get medieval, I'm goin' in for guns"</b> - <i>Dos Gringos</i>
I'd like to see NASA team up with the Thunderbolt designers and see what really neat improvements could be made on this true classic. No conventional rules apply, open the thought box. Dare to evolve the impossible.
Maybe a mini-F-23 shaped wing with a blended root bulb for added internal fuel... and try all-moving F-23 style canted tail? Maybe convert to an F-20 style w/ enhanced Volvo RM 12 engine pipe?