That kit ain't plug and play. You are going to have to cut all the intercooler piping yourself since it's a universal kit. And those manifolds and Down Pipes are notorious for not fitting, Cracking, or both.
slevin, i see you have the greddy kit. Are those kits "plug and play"? From pics it looks like it doesn't come with enough to make it "P and P". Do they come with fuel managment? Beyond how much the greddy kit actually costs how much else would I spend to make it a significant power gain?
i would recommend cruising miataturbo.net. usually a greddy kit will pop up every once in awhile.
i got my kit over there for i think $1200 with intercooler, pipes, heat shield, RC 440 injectors, boost and a-f ratio gauge.
the stock kit comes with a vortech FMU, it;ll increase fuel pressure gradually with the signal line in the intake manifold.
honestly when i first installed the greddy kit and maintained the stock boost it was really fun. the stock kit makes a decent amount of power, but after i upgraded (intercooler, manuel boost controller, 1.8L injectors, and 2.5" turbo exhaust) the difference was huge and very gratifying.
my brother currently has a totally stock greddy kit installed on his miata. the only complaint i really have about it is the relocation of the stock air filter and how close the downpipe gets to the heat hose.
i cant really tag a number on how much more your going to need to spend to get a huge jump in hp, but i hope this will help you.