IDK about your question as i am in the middle of the quadery of life.
I know you can get more RPM out of the sporty....more air equals more gas
How ever- everything i have studied has stated for real performance 2 into 1 is the ticket.
That said i used to have a 76 iron head.
(Sold it to someone in kent)
I put 10.5 CR in it and a few other cheap adjustments.
It flew.
I could beat the big bad 900cc Kawi back in the day.

Jeff, you could buy the Thunderjet from Zippers and do the mod your self, or send it to them and have them do it. They can also add an adjustable air bleed to help with the mid-range tuning of what used to be the main jet circuit. Once properly set-up will cure all this and run like a scalded dog at all RPM's

I would keep the cycle shack pipes for now as open drag pipes tend to make it peaky......................