all depends on the car, if you're set on a miata, check out some mazda forums, search there if the engines can hold up after 100k(i'm sure it'll be fine, as long as you get enough fuel ;-) )
my buddy +t'd his car with 200k, and he doubled his hp and got even more torque for $600 invested(including programable ems)
i've got pretty much the same setup and my car has over 300k. perfect compression on all four cyls
usually the motor can take the boost, but the drivetrain can't take the powa
later man
jason