I expect 991 Turbo will have performance even further away from the 458
Sorry but performance is much more than just a straight line drag race which is what the Turbo excels at. Round a circuit the 458 leave the Turbo for dead check out the EVO test times.
One of the reason the new 991 is quick is that it is lighter and they have widened the front track reducing understeer and making the car much quicker through corners.
I noticed a big difference in weight and cornering performance when I switched from a Turbo to a GT3. I hope the new Turbo will be much lighter as that will make it a better drivers car and more fun but £150k is till a lot of money for a 911