[TheMAN]

you want to improve fuel efficiency? run the lightest motor oil possible... there are MX-5 guys running 5W20 or 0W20 synthetic in the BP here in the US, in the hot southern part of the country.... unpredictable oil consumption at times but no appreciable motor wear or damage from analyses reports... to be on the safe side, a 0W30 synthetic will work wonderfully with this engine

use synthetic gear oil
run higher tyre pressures, use harder rubber compounds
use lighter wheels with less open faces as said
fit ground effects and lower the car to reduce coefficient of lift
fit air diffusers under the car to reduce coefficient of lift, air turbulence, and drag

remove the passenger side mirror if that is allowed... or both if allowed
remove boot lid spoiler (you did that)
remove or put away the aerial

if you decide not to run aircon, you must keep the windows up
having the windows down negates the advantages