Speak to BBR and Roddisons motorsport if you want to tune the mk3.
Intake and basic exhaust mods (backbox only or mid pipe and backbox) do very little for power. Intake mods hardly make amy difference to the sound either and the stock intake is not restrictive. Best value for money is add a pipercross panel filter and then spend money on manifold and remap and maybe exhaust if you want a fruitier soundtrack. A long arm intake like the AEM one does give lower intake temperatures due to the filter location but the change in performance is small, maybe slightly sharper throttle response- I know cos I have one.
The ecutek remaps are good, can gain up to 10bhp on a stock 2.0 car but the real improvements are more apparent than this. As standard the throttle map on all mk3 mx5s is dulled to try and present the traction control kicking in, for insurance grouping reasons and to protect the inexperienced driver who may have an mx5 as their first rwd car. In the first three gears the speed at which the throttle plate opens when you boot it is deliberately slow, this makes it harder to light up the wheels but also makes it much less responsive and generally feel lazy to drive. At certain speeds in certain gears, planting your foot all the way on the accelerator does not give 100% opening of the throttle plate for a full 4 seconds which really dulls the drive. The remap removes all this nannying and the car feels far sharper to drive as a result.
A 4-1 exhaust manifold needs a remap to run properly without leaning put the fuelling and throwing up a check engine light, but with the remap and manifold together with a good exhaust and intake you can acheive approx 180bhp on a mk3 2.0 and 185bhp on the facelift cars as the cylimder head is better and rev limit slightly higher (these are 09 plate onwards cars). If you do change the manifold or exhaust mid pipe you need to consider emissions requirements and noise too so choosings parts carefully is vital to ensure your car will pass the MOT and not be stupidly loud.
There are also supercharger and turbo options but very pricey however 240bhp from the 1.8 is possible and 330+bhp with a forged engine and stage 3 bbr turbo kit etc.
The 1.8 responds to tuning about as well as the 2.0 (e.g. +15bhp for remap and exhaust) but obvious it will always be slightly down on power, and the benefit of the remap (if you have this done in isolation) is perhaps slightly less obvious, as the lower powered car doesnt have quite such a nannying throttle map anyway.
