Infact I have 2 MX5s.
A red 2007 NC 2.0 and last year I splurged and bought a 2018 ND 2.0
I had the NC fitted with LPG (Why - not especially thirsty but was 79p a litre for LPG at the time - so why not !). Its also had coil-overs fitted. The huge gap in the wheel arch was bugging me
Last year I splurged and got myself a 2018 2.0 model.
Done nothing to this but drive it.
Tax doesnt change. Performance is the same - as close as make no difference. Only real time you notice is when you run out of LPG. If you happen too be gunning it it misses a beat as it switches back to petrol and carries on (with a beeping noise to remind you that theres no LPG left - a button press silences this).
The downside of course is the boot has almost no space left. But since it wasnt huge anyway and I was using for my daily commute this didnt matter.
I was getting around 120-140 miles from a single tank of LPG - which was around ÂŁ20