I ran the ST G.15 in A speed in the middle '60's. Nice engines, quite fast (if setup right) for the day. The K&B is the same model ('67, I think, wedge head from casting) I pitted in Rat. It'd go 134 with a trimmed 9-8 Tornado nylon and 40%, otherwise stock. It started so good that we didn't need to practise much. My pilot flew Navy Carrier, and could put the plane down in the right place. I ran the ST .29rv in B and B Proto, but it was a lapped, flat topped piston, while that .40 you have should be a baffled piston, probably ringed. I'd expect the ST would be on par with the K&B, but last longer. We had to replace the piston/ring/rod/cylinder every winter on the K&B.
There is a move afoot for a Vintage FAI FF class...'60's F1C's would be great fun...for that G.15...if you have a place to fly them.
Steve