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As long as the rolling diameter of the tyre remains the same, any wheel and tyre combo can be applied (within limits obviously). Handy little calculator I've found: http://www.miata.net/garage/tirecalc.html

Alloy wheels would have been optional across the range but would have come standard in everything but your pov pack... Not sure, seems the way the third gens work though.

know my car (05 TW Wagon) came with 205/65/15 steelies. what are the tire size on the optional mags i know they are 16"x6.5 so would that make them 215/60/16 to keep the speedo size the same?
actually it would either be a 225/55/16 or a 215/55/16. a 215/60/16 would be a bigger difference in tyre size than the 215/55, and the speedo would under read (you'd be going faster than what the speedo said) where as the other 2 will over read slightly, so you would have less chance of getting booked for speeding, as well as having a sportier looking tyre, less tyre roll when cornering, and lowering your ride height ever so slightly.

Been doing a little research into this myself the past few days.
215/60/16 is spot on for a 3rd gen, what reads on your speedo is how fast you are going.
The standard 205/65/15 reads 5% over, so at 100km/h you are actually only doing 95km/h.

well if 205/65/15 tyres are 5% out then 215/60/16 will take it down to about 2.5%. it won't take it to perfect. but it really comes down to brand of tyre, and tread depth. your speedo error changes as your tyres wear. and different brand tyres will have minor differences in sidewall height, which also affects your speedo.

another major factor is tyre pressure cause a tyre isn't round unless its fully inflated.

Is it this one?

This is a factory 16" (running 215/60) off TW AWD, also fitted to TW Solara.

Oh ok, Anyway I reckon 16's look too small on any Magna and will probably go 18's when I have the spare cash.