I've been breaking in one of these on a combat plane.  It is a little hummer.  The sleeve is ported a bit high(148 deg exhaust), but the shaft timing is very much in line with stunt numbers(190 deg duration, close at 45 ATDC).  It's starting to come in now and runs nice and smooth.  With an open exhaust and intake it turns a 6.5/3.7 APC around 23,000 rpm and pulls the plane similar to an old Black Head Profi with no muffler.

We'll try it some more for combat.  I'd like to see how it does on 60 ft lines for speed limit.  Then we'll try it on some larger props like a 7/3 or so.  Final stage will be to open the shaft timing up to close at 55 deg. and see if anything breaks.

If you're in the market for a nice 15 this looks to be a good one, like its bigger brother the 36.  For stunt/sport use you will have to come up with a suction venturi and a needle valve assembly. 

The last year or so, Currell Pattie had a set of "20" size Nobler plans (along with some other like sized planes from the Orient.)  it is different from the Jr. Nobler and looks much more like a '57 GB IMHO.

Maybe would be a nice candidate for the TT 15.

The Original Veco Tomahawk also comes to mind.  A pretty underrated profile that flys much much better than expected.  There are some differences in it and the Bordak kit since I have acquired a set of "plans" for the Veco and compared them.