The Tesco 99 fuel... is part Bioethanol...
Now the only benefit the higher Octaine rating is going to give you is the ability to advance the timing without causing "knock" or "pre-detonation" and the ability to run higher compression.
This is also why Turbo engines when "on the turbo" will benefit (they also have nock knock sensors).
Additionally the Bioethanol part of the Tesco mix causes a slight lowering of the intake temperature at the port inlet. For every 1 Degree C you should see something like 2-3% BHP increase (roughly speaking)
The only downside is that Bioethanol has a lower chemical energy content therefore whilst providing some knock protection and reduced intake temperatures it does not necessarily provide as much chemical energy as a 100% unleaded mix such as BP105.
At the end of the day.. even with an adaptive ECU it will still be within a boundary and chances are you'll only see major benefits with a re-map.
On the most part the, car will feel more responsive "chrisper" and the engine will run slightly smoother (Although this can vary significantly between fuels)...
Probably easier to find one fuel that make the car "feel" and run well and stick to that where possible... for performance it's not going to make the biggest difference... especially for the cost of purchasing High Octaine fuel all the time.
