Getting appropriate squish on a true hemisphere is not so easy (precise) as one has to factor in the lateral slop movement of the piston within the bore. Have you ever rocked a piston left and right in a bore along the axis of the wrist pin? Now think about the piston dome and combustion chamber wall geometry; what happens to your squish while your piston can rock and slap forward & aft as well as side to side and you have a nearer to vertical clearance between the piston dome and combustion chamber. One side near closes up and/or hits and the other side becomes sub marginal or ineffective squish. I seem to recall the Ducati mono cylinders have a nice compromise approach with a gradual (nearer to flat) dome piston and sloped squish to match on the sides; at least it looks pretty. This brings up the question on how the HDXR750 managed squish. Maybe I am missing something here.
As for a Commando 750 being superior to a Triumph 750, though squish may certainly be a factor in this I believe it is the demonstrated superior intake port efficient flow characteristics of a Commando that make the big difference which naturally translates to greater volumetric efficiency. The "why" of this better flow is a topic of another thread. Furthermore, when Norton splayed the exhaust ports, the intake ports were rotated about the axis of their respective valve stems in order for the two ports to be parallel. This new orientation of the intake ports to the center-line of the cylinders imparts considerable swirl to the intake charge. I have not run a flow bench swirl test of this but I am stating what appears to be obvious. When one looks at the true orientation of the port to cylinder center-line, the intake flow comes in at a considerable tangent to the cylinder rather than straight in like on a Triumph.
Efficient flow and turbulence are good and are why a Commando generally can out perform a Triumph of that era.