Originally Posted by dougnash

For a naturally aspirated engine, I might imagine so. But, and not that I'm more experienced than C&D's research staff, but the advantage that the forced-induction car has over the NA one, at altitude, is the ability to constantly adjust the level of compression. Where a NA car will rapidly lose compression (and therefore HP) as the altitude climbs, the much lower cylinder pressure would arguably allow for a lower-octane fuel.

But the turbo-charged engine would keep the bypass valve closed more, and more - thus, boosting pressure and heat. It would (up to a point) be able to mitigate the high altitude, by approximating a lower altitude with denser air. Get it? Therefore, you're going to want an appropriately detonation-resistant fuel, to go with that.

Don't take this info from me, nor a commercial car rag. Just do some extra reading in engineering and racing books / web sources, and it'll explain a great deal.