AFAIK there aren't many £5k frames, in fact the Cervelo RCA frame set at £6.5k is the only one I can think of.
In reality a lot of bikes in a range often have the same frame and it is the components that increase the price as the range move up the scale, so it is mostly likely any comparison would be between different manufacturers or different types of bikes, endurance, aero, lightweight etc, making a speed comparison tricky.
It would be great to have a way of accurately putting out a stable 200watts on a range of bikes in a test across an identical course to then actually be able to measure impact on speed. But as has been said, speed is achieved by factors other than solely power and aero, such as the riders ability to put down power potentially influenced by the comfort of rider, over both short and long term. I guess this is why a fact based comparison isn't as easy and it often comes down to weight and a load of bulls#%^ bingo when it comes to reviews.
Anyway, this is Cyclechat, where the cheaper the faster and anyone spending more than £50 on a bike is a mug with more money than sense