"I have just made up some fresh "PMC" that was one of your earlier formulas. I mixed it without bromide, and with 25g/L metol, and got a healthy 200 speed out of HP5+."
Interesting that you found that information from so long ago. PMC stood for pyrocatechin/metol/carbonate, but it sounded too much like PMK. There is no practical working difference between PMC and the current Pyrocat-HD with metol. The PMC formula called for sodium carbonate and not potassium carbonate, but so too did early versions of Pyrocat-HD. It really makes no difference which of the carbonates you use so long at you get the same amount by weight into the working solution.
"Could you explain why bromide gives more speed? Would varying bromide change acutance effects?"
Bromide is a restrainer and when used in the right amount reduces fog without cutting speed. However, if you use too much bromide the result will be not only very fog free negatives but also a signficant reduction in effective emulsion speed.
"I'm particularly interested in semi-stand development with half-strength pyrocat-HD. Even my faulty batch gave me brilliant, lively sharpness.
You might want to look at the threads at the AZO forum on the use of Pyrocat-HD with semi-stand and minimal agitation. Quite a number of peopole have been experimenting along these lines and reporting their results.