Andy Wana wrote:118 - 123rwkw from a Naturally Aspirated engine sounds great.
What sort of mods (and money) are involved to get those kind of numbers?
Lots of extensive mods and a sheetload of money. I'll let CT explain... Although, he has done so before when the same question was asked, so a search will probably give you some good documentation.
The IRTBs interest me (as much as the turbos) because somewhere in the back of my mind, I wanted to keep the NA character of the MX5 should I choose to go for more power
The NA character is brilliant. I loved the feel of the car before, it was just a little wanting when overtaking and going up hills...
In my humble opinion, trying to wring out as much KW out of the engine on dyno days via turbo/supercharging is like a "my manhood is longer than yours" competition
What if you went turbo for the power increase, and not for the bragging rights? You can have manhood competitions no matter what way you go. I have to say though, turning up to a dyno in an MX5 and expecting to show off the size of your manhood is a bad idea from the start...

It won't end till you end up with a handful weekend dragster rather than a driveable daily roadster in your garage
You have this the wrong way round. Wringing the neck out of NA engines is where you are going to have something hairy that you can't use as a daily driver. The more top-end power you aim for with a turbo, the more dull (increased turbo lag) it feels down low. Turbos are easy to control. You run in vac when the throttle plate is closed and boost can be controlled with your right foot.
It seems that trying to wring out as much fun as possible whilst getting more rwkw would be the go for me (i.e unbelivable throttle response, quad sounds, willingness to go into the redline)
ECU, cams, free-flowing exhaust, quads, light flywheel. It would be a bucket of fun.

Pain in the arse in traffic though.
There are benifits both ways. I would not recommend "wringing the neck" of an NA engine though, if the car is primarily for the street.
Boags