Theoretically there shouldn't be much between the performance you get from Tesco 99 or Shell V-Power as both have the same RON rating.
And whilst the theory is correct, reality actually proves very different. 
The difference will come over a longer period of time.
Huh, please explain! 
Running on Shell V-Power - The reports I've heard are that the RON rating is much more stable and that the fuel has more additives in it to keep the engine healthier.
You got that the wrong way round. It is the Tesco 99 which has been repeatedly proven to be more stable. Thorney Motorsports have systematically proven this, as have Revo. V-Power is much less stable than Tesco 99 - the only time it is on a par with Tesco is when it gets delivered by the bulk tanker.
Tesco runs a larger element of bio-fuel in it's make-up and so could be an issue for the FSI engines over the longer term in terms of keeping the engine clean and longer term maintenance of the power outputs due to build up of deposits. 
That is wrong too. By law, ALL UK and EU unleadeds, weather poverty spec 95 RON or BPs 102 Ultimate - all have to have 5% bio fuels. But being as GreenErgy, who make the Tesco 99 brew have been using bio fuel base-stocks for years - they have far more 'bio ethanol' experience than the traditional 'crude oil' based oil companies. And Tesco 99 uses an advanced additives package, which is on par with any other UK fuel, super unleaded or otherwise.
V-power has more additives in than Tesco to help keep the deposits off the valves etc.....or at least that is what they purport! 
Then Shell are feeding you mushrooms - Tesco 99 has an highly advanced additives package, easily equal to what the 'major' oil companies claim. What's more, log onto
www.GreenErgy.co.uk, click on the contact us button, ask them what additives they use, and they will e-mail you back with their full additives package. Shell, BP, and the others certainly don't do this - they are more than happy to spout about what they claim, yet completely fail to back up their 'claims'!

Seriously, Tesco 99 is categorically the best 'pump fuel' available in the UK. OK, there is the BP Ultimate 102, and when it is fresh, it is superb. But at just 10 stations around the country, it is hardly gonna be 'local', and it suffers from horrendous degredation - for £2.50 per litre!!!

I thought that V-Power was the more stable of the two?!?
Nope, other way round. Ask Revo, and ask Thorney.
Are you telling me Tesco 99 doesn't have more bio-fuel in it's make up?
Tesco 99 has EXACTLY the same bio fuel content as V-Power, as well as every other 'petroleum-based' unleaded. But GreenErgy supplied Tesco fuels had been using bio-fuels for many years before the likes of Shell added them, and so have a massive 'head start. And don't forget, GreenErgy are the UKs third largest petrol retailer - so they ain't no 'cottage industry' oil company!
HTH