The A15 piston has a 30mm pin to crown height, and the A14 has a 32.5mm height. The mazda E5 is 29.1mm, hence the need to deck the block. Although the mazda pistons have a much smaller dish, despite the lower crown height. Also the standard diameter of the E5 piston is 77mm, so you'd need to bore your block 1mm over right from the get go. As well as the conrods needing to have the small end opened up to 20mm (from 19mm)to take the mazda gudgeon pin.
Perhaps price up new pistons before deciding which way to go? E5's might be false economy by the time you have your rods modified? Although you will get a higher CR..........