From what I know, if you're boosting it you'd have to use the MSP ECU. The MP3 is what you want for best NA power.
if it's well taken care for and in good shape, boost the stock engine. install an oil pressure gauge beforehand to see where it stands and get someone to do a compression test.
I just ordered a compression tester. How hard is it to install an oil pressure gauge?
... from what I hear, the stock one doesn't trigger the oil light on the dash until it hits 5psi or something low like that. basically too late by the time you see the light.
Here's a link to Boosting your P5...
https://www.mazdas247.com/forum/sho...ng-your-P5-Here-s-a-guide-for-getting-started