I had Group K port a 750 SS cylinder, deck the cylinder, mill a 750 head, nozzle boring, mod a 759 single manifold for a Keihin 42, and machine the 750 SS cases for a 650 stator about 5 years ago. Well after it seized they were well let's say far less han helpful in diagonising my problem. Left a bad taste in my mouth so I won't use them again. I know another member on here had a similar experience.
Pete used Rober Sand aka Jet Ski Mechanic for his first 800+ cc X2 build. You will have to ask him his opinions on the work. I know he had new sleeves put in that engine.
Me, I am still depating on who will build my next big bore 750 engine. I know Top End performance used to do all of Kawasaki's race SXi Pro engines. I want a freestyle motor though.