I did the head on my S2 a few years ago, mainly because i was bored
I was already running a bar of boost, and had rebuilt the bottom end the previous year with some fancy bits, all lightened and balanced too. (Still on MFI)
Andy Weston the headwork, and although i didnt go for one of his "Ultimate RS Turbo" heads i had the maximum he could do without bigger valves. He also fitted a Kent CVH35 IIRC.
For £700 odd outlay on the head, plus gaskets, bolts etc that really made it the thick end of a grand, all i gained was a whopping 7bhp and shoved the useable power further up the rev range. Yay!
I'd like to think i need to be running more power to make the most of it. For what i wanted it for, it wasnt worth it TBH.

Incidentally i just rebuilt another engine for another RST, and one thing you can do yourself is to dowel and match the inlet / exhaust ports. The one i've just done was shocking! The head ports overlapped the bottom right quadrant of the inlet ports by a good 5mm... Soon put that right!