Honestly, I would never paint my MSP anything but the stock color. However, MSP2024 knows the truth. We (at least I) are a very calm people. I personally love the flat black color, and especially the pic you've posted of the color you want. However, IMO I would get it all done, engine bay and everything. Just guessing but I am going to say that you'll paint the entire car like you've described. Then, when someone wants to see what's under the hood, you'll get that gut wrenching feeling like "Damn this guy is going to think I am a ricer." You don't want that. I'd save up for the entire job if I were you bro.
Of course, this coming from the guy who wants to send his MSP back to Japan and tell those bitches to make it like it came from the factory (but with the KLZE-Turbo swap instead of the crappy FS-DET).
My final word... the only color I think you'd be doing a disservice to, is the Spicy Orange. All the other colors are pretty standard Mazda colors that could easily be put back.