I phoned Paul Ivey today to chat about high flow valves. He is pretty sure that on an engine with my spec I'd see at least a 5bhp gain. No I feel this estimate to be a bit high for mere high flow valves. But if I do get 5bhp extra then I'd be well happy - pitty I'll never know though, as other mods are being done at the same time.
I'm going for standard stem size as I forgot going to 6mm would also require new sping caps and collets - doh!
I'm getting bronze valve guides fitted by Cylinder Head Engineering in Fareham. They will also recut the valve seat and redo the bodged Regal attempt at three angles of the seat. They will do 4-5 angles in the inlet and three on the exhuast. They will also do three radiused angles on the valves which will further increase power.
Going by Paul Ivey's estimate along with the standard gains from reprofiling, I think it is likely that I will see that gains expected once the rebuild is done (i.e. with the new cams too).