Ah you guys aren't thinking this out straight.

First of all, the A15 stroke is very short compared to other hi-revving engines.

Secondly, the bore-to-stroke ratio is more interesting in its effects.

Lastly, as mentioned, there is no replacement for displacement.

From the half-logic I keep hearing one might begin to think the A10 is the best "peformance" a-motor -- since it has the shortest stroke!

One of the reasons the A14 is favored by racers is because of class rules. Which doesn't apply to street cars.

The A15 head has the same size ports as the A12 GX head. The biggest effect on feel is the torque comes on sooner due to the bigger CCs, then levels off sooner as it's flowing more air than an A14. it's partly psychological and partly due to the factory tune -- unless anyone's taken objective measurements (for example, timed both 0-100 km) this is all just guessing as to whether a stock A14 is "faster" than a stock A15.

Getting adjustable cam gears and advancing [edit: retarding] it a little should perk an A15 up in the high-rpm section (has anyone tried this?) along with advancing the dizzy timing and richening the jets a little. The late 70s motors generally were a little weaker in those areas to gain lower emissions. Disconecting the EGR valve and air pump should also help performance.