Bike looks real nice I was thinking of running a JP 9 piston in my VCH but I don't have an estimated weight for it, the picture below is my current one which weighs 485 gm and the pin 113gm. Would be interested what yours weighs as my old piston (the one with the hole was weighed when we still had ounces I don't remember, what it was which was shame as I balanced the crank for that piston, bit late now it has been lightened in an orthodox manner)
I did have a BTH mag and have copies of the service manual which seem to indicate the gear ratio is the limiting factor for rev range. I now have a Lucas SR1 and it ran fine at 7500 rpm, I don't what the specs are but maybe the lower speed limit for the BTH comp is based on testing where they look for a maximum number of "missfires by the magneto" on a test rig of 5%." these may not equate to engine missfires and at 100mph with a rigid frame you don't seem to notice much anyway.
Below is my two pistons, would also be interested how large the dome and valve cutaways were on the JP piston.
Keep us posted hows it runs on the beach.