[RICE RACING]

I would look at it this way:

It makes much more sense in a knock limited boosted engine to have the leading spark plug lower as when it fires it is more central to the combustion pocket = better power and more efficient.

For a N/A motor with more leading spark lead it makes more sense to have the spark plug higher as it will be more central.... and will deliver more power and be more efficient.

It would be related to the spark timing that is tolerable as to where to best position the leading spark plug. Would not loose too much sleep over it, Mazda are smart enough cuuuuuunnnnnttts and power and being efficient go hand in hand for the application.