Yes, the A14 feels significantly more powerful than the A12. If it is in good condition, you won't be disappointed. I wouldn't say it's a case of "wow", but more like "that's nice!". To me it is worth $100, but I wouldn't use the small-port A12 head on an A14. Here in the US the A12, A14 and A15 all have about the same compression ratio, so C/R is not a factor of my experience.

The stock A15 has noticeably more low-speed horespower than the stock A14. I've had all three engines in stock form in my 1200.

Now, getting away from personal experience and back to theory and commonly held wisdom: If an A14 and A15 use the same cylinder head, they will have more or less the same peak HP -- the head being the limit to airflow. There is theory about rod/stroke ratio that favors the A14, but we are talking about something like 1 horespower difference. The A15 is only good to rev up to 8500 rpm or so (with upgraded valve springs), but as pointed out, the A14 can apparently rev even higher. If anyone has dyno tests or dragstrip times (personal experience) that shows different, let them speak up now. This talk about A14 vs. A15 is almost all theory, and few people have built up A15s -- those who have generally report very good results, to at least match the A14.