Listen to those guys above. You need standalone engine management with that much power... and you don't want to mess with the Greddy kit. FlyinMiata makes a plug in standalone called a Link which is the easiest plug and go unit. Also consider Emanage Ultimate, Hydra, Megasquirt and a few others. If you're looking for ease of install, go with one of the FM or Begi kits and a water injection kit. With a built motor, you should be able to pull 260rwhp running in the 15-16psi range depending on turbo. I'd also consider water injection. Perhaps a T3/T4... maybe a T28? You could build your own kit for cheaper and have better performance though.
However, why have you chosen 260rwhp? Is that an arbitrary number or are you shooting for somethihg specific. Assuming 15% loss, thats about 310hp@motor. Remember, the car only weighs about 2200lbs. That much power will give you around 7lbs/HP. For comparison, a 2001 Z06 was 8.1lbs/HP and a new Z06 runs 6.2lbs/HP. Did you know you were talking about that much power? Also, the stock rear dif of the 1.6 motor will not handle that much power. You'll snap it backing out of the driveway. A used 1.8 Torsen upgrade will run you about $700-$1000 depending on source.
You should also consider a serious brake upgrade... I'd look at www.good-win-racing.com
and his brake packages.
Off the top of my head for other things to consider.
Cooling: the stock radiator will be insufficient to cool that much power.
Suspension: replace shocks and sways at a minimum.
Tires/Wheels: you're going to need some wide rubber to put that power down.
Safety: Rollbar... think about it hard.
If you've got most of this covered already, just fill in your sig or at least the text part of your vbGarage. And welcome to FI... I'm new myself and have already roasted the stock clutch... SpecII install to commence this week!