All S2000's can run on 95, I've always used 97 then Optimax, with OPTIMAX I get 2-3 more mpg, (~<10%) so there is the saving over the greater cost (6p at the moment) I think you might see more oomph over 95, but perhaps not as much over 97.

No brainer for me just on the maths issue!

 

I use Millers Octane Booster! great stuff, and it does work. I don't use it every day, only on those special, but needed occasions.

I use it in a combination with Optimax to increase the RON to about 100+. You can get Millers for about

 

Mine is noticeably smoother on the Optimax with less hesitancy when pulling away.
I resent paying the extra money but I'll do it because I can see the difference every time I don't do it.