Compression comes from the piston moving to create less volume in the cylinder/combustion chamber. The rings and oil film create the seal between the piston and cylinder. The taper is there to control the dimensional characteristic of the cylinder so that the cylinder is straight during operating conditions (temperature) A ringless aluminum piston does NOT rely on the taper to seal. It is the film strength that results in the seal. This demands great accuracy in control of the piston to cylinder clearance. And it is this demand for accuracy that make the breaking of ringless systems so critical (no rings to take up the slop).
In an ABC/ABN system the tapers and interference (pinch) control (address) different concerns with regard to drag and sealing ability at operation temperatures. Now most of this is controlled at the point of manufacture so one has little control over these. But one can control the break in (stress relief {thermal or machine}, work hardening) and as such should be done on the bench to avoid a lean run during this critical phase.
Now many high performance engines were available with different size pistons to take into account the various fuels and power setting one might expect the engine to perform at. No one fit is suitable for all applications.



Konrad
FYI: I set up all my full sized air cooled car engines (2.0L to 3,3L) with cylinder taper and these also had rings. Again this was to control the difference in size at the top of the cylinder as a result of heat.