Valve size alone does not determine performance, and neither does compression. Its all about the total package. Personally I feel 7:1 seems rather anemic, but what do I know?
I know a guy up in BC Canada who helped get TRIBSAs legal and into a popular race class in AHRMA MX racing here in the US. (He is from UK Originally and has a lot of race history including ISDT).
So Ken, and a few others finally got the race organization to allow the TRIBSAs and he and others went to work feverishly building what they thought would be the ultimate race bikes.
A10 SA frames, Preunit Triumph motors and most wanted the 500cc twins, although some went big into 750 big bores (Different classes). In Kens case got the alloy close pitch fin cyls and heads and then had head work done, and HIGH Compression pistons and big lumpy cams.
It was a beast, snarled and bucked, fearsome.
However he had a lot of trouble with it, It pulled out cyl base studs and other hardware issues, and basically every race outing was a DNF or serious other mechanical problems. From a variety of inputs he finally concluded to redo it and De-Tuned the motor, Lowered the compression, different cams and it turned out in the lower state of tune it had more than enough power but importantly was instead of raw power he finally dialed it into low end grunt and mid range that was more useful, especially on West coast tracks where high speed was not a factor. Most importantly the bike didnt try to kill itself and he started winning races.
When I was in my 20s I owned 2 Chevy Hot rods, Both Novas, a 1972 Rally Nova and a 1964 SS. I was convinced after years of research and all the popular magazines that bigger was better and built a variety of Small blocks trying to push the envelope, Big cams, big valves and heads and all the Go-fast parts my meager budget would allow. I finally got the opportunity to get a Big Block motor and it was a 454 and rectangle port heads and a variety of performance parts on it. It WAS fun! And had more power than any person could use on the street. But it was VERY expensive to build, and run, not to mention it had poor manners in driving. I actually took a young lady out in a date in it and she concluded I was an idiot, why would you want to own a car like that? (Lets just say City driving was a challenge). (There is a substantial weight gain with a BB over a SB as well)
In my daily driver hotrod, the 63 SS I had a 283 SBC and I learned to love that motor. For a small car it had a ton of power, I could beat many other cars on the road, but behaved nicely in any traffic or conditions. It had stock compression, very small valves and the intake ports in the heads and manifold were small as well.
What I learned was for low end USEFUL power, it was about velocity, not mass, Small was better. Chevrolet knew this as well and this was called the "Power Pak" option. While I still love building big motors, The little 283 in a light weight car is one of the best versions out there. Some racers knew this as well and slightly tweaked them into 302 and 305 configurations and they dominated SCCA Trans AM sports car racing in the 1960s and early 70s.
I would surmise that this head with the small valves would be interesting to do some testing with, as well as try with a variety of cams and I bet in the right configuration of cams, compression and carb (Assuming timing and spark quality are optimized) that this could be a very powerful engine for certain applications. I doubt anyone will do Trials bike riding with a PU BSA Twin, But dialed in right it could be a potent street or Dirt racer. Dont overlook the exhaust pipes, They need to be optimized for this configuration as well.
Much like the 283 Power pak vs a 454 BB, I would look into it before hogging out the head and changing parts. Many of the best and brightest who used to build performance versions of these motors are either deceased or one foot in the grave, So not a lot of resources to ask anymore, but perhaps some documentation exists on alternatives to monster motors.