From what I've seen and heard, the worst affected Turbos are those that have been driven on V-Power. Just doesn't seem to come back to you.
A colleague of mine also has Turbo and he also moved from Shell to Total after my recommendation and its knocking just hasn't improved. He keeps using Total HOBC 100% - somehow I think you should not use more than 75% HOBC. The best my car has felt (other than the first 500kms) has been when I've gone with around 60% or so HOBC and rest petrol (Total of course). Reason I 'rolled back' from 100% HOBC was that I observed more than acceptable (for me) amount of carbon on the exhaust pipes - so I thought what the hell, lets roll back and sure enough the car responded positively to that change. I haven't used 100% HOBC for atleast the past 8-9 tanks.
I know its hard to move down from 100% HOBC to a mix when your car is knocking, but that's just my personal experience.