I'd say the cost will be about the same, they're just two different routes with one more cost effective per BHP than the other. Both will come with an ME221, which is the bees knees for plug-and-play ecus for the MX-5. The throttle bodies apparently sound superb, and you get more readily available power, and as a bonus the engine bay gets a lot tidier as you lose all the air intake gubbins on the right hand side and front of the engine. Power is about 175bhp but it's an expensive way to get 40ish bhp over a standard healthy engine, as you need to port and polish the head and stick in uprated cams. Turbocharging meanwhile gives you more power for about the same money, but you get higher temperatures to sort out and power isn't quite as readily available. I doubt on an MX-5 it's going to make much difference though. I'll definitely need to fit an LSD, although that's not going to be too difficult. Reliability shouldn't be a problem, and my engine appears to have been looked after well...