According to a recent test published in Banzai magazine, it looks like Tesco's 99 stuff is only marginally better than 95.
Their test was done on a proper engine dyno using a highly modified Nissan 200sx engine and stand alone ECU. The test showed that 95 octane needed the ignition timing backed off and it produced just over 500bhp. Next up was the Tesco stuff, and whilst they could re-advance the timing, they couldn't add too much more before det occured. Tescos stuff made about 510ish.
Next up was the BP ultimate, which allowed a further increase in ignition timing and made around 525ish, and lastly, Optimax whch allowed even more ignition advance and pushed out about 537.*
So, providing you have nice fresh Optimax, it still looks like its the better option. Although Optimax can lose a RON or two as time goes by, so I'd imagine those results were based on fresh Optimax.
*Don't quote me on those figures. The magazine was published a good few months ago, and my memory isn't THAT good;)