Thanks Downhill H.
I might have mentioned in a previous post that I did not change any carb settings once I put the expansion chamber on, because the Motocompo comes with a pissy 12mm Keihin carb with a pressed in main jet. As I never intended to persist with that carb, I wasn't inclined to run a drill through my original, incase I ever wanted to revert back to standard some day. But I completely agree that it would have gone better still with some more fuelling.
Anyhow, I actually got this beast fired up over the weekend with the 70cc Polini and 21mm Dellorto in place.
Unfortunately (or fortunately actually) my jetting was well on the conservative side, with the bike 4 stroking crazily at around 6,000rpm. Even so, it surged to 25mph quite quickly, and should do a lot better once I can access those final 3,000+rpms. For reference, I started with a 98 main jet.
However, I have not pulled it apart just yet to downjet, as my idle jet is also too high (carb supplied with a size 50). I just ordered a 42, 45 and 48 to play with, so once they arrive I will also downjet the main and try again!