knock control and mixture control are quite separate processes in theory, but they overlap because they can each affect the other.
the lambda sensors job is to monitor excess oxygen in the exhaust and hence determine the AFR (air-fuel ratio), and in the case of narrow band sensors, simply rich or lean with very little determination in between.
the knock sensor, in essence, monitors the quality of combustion - but only in so far as it will pick up the knock signal present when the mixture starts to pre-ignite - other than that, it provides no input to the ECU other than 'no knock detected'
it makes perfect sense that higher RON fuels are beneficial to forced induction engines as they are typically running with a much greater flexibility wrt fuel types - the conditions inside a turbo engine are much more variable than normally aspirated so the management has to be able to cope with this - so as well as being able to retard the ignition on a hot day, it's also able to advance it on a cool damp night, or when it detects that the fuel quality is better - or both of course.
aspirated engines are far more linear and predictable, even under wildely varying conditions - so other they will have the same knock control, the net variability either way is much smaller, so just the same as you will not notice much advantage when running a fuel with a couple of points better RON, you'll not see performance dropping off so much either.
this is way oversimplified mind - and really only addresses knock etc as a single entity - in actuality like I said, mixture also has a huge part to play, and the above only really holds true for standard factory engines and their managements - once you start getting into aftermarket management and tuning, and proper wideband lambda control, and throw the economy/emissions book out the window - it's a whole different ballgame. For instance excess fuel is actually used to cool the engine and stave off knock/detonation under full load conditions - AFRs as low as 11:1 are not uncommon (if you've ever seen a full on turbo engine under full load and wondered why it's smoking like it's going to burst - that's the excess fuel which is actually preventing it from doing so - also the reason why you see the spectacular flame outs)
exhaust gas temperatures are another important monitoring point in turbo engines - they are another indicator of the current state of the engine - if they climb too high they'll also set off detonation etc.
I'm not surprised your RS4 is quicker with a better fuel - any percentage of 380bhp is naturally going to be more noticable than 200 or so.
other than reading up on FSI, I've no more knowledge of the A2 engines - other than doing cylinder heads for people, I've told myself no more mucking around with engines! Until such time as I buy something more interesting, the A2 and A4 are strictly off limits!
I nearly put down a deposit on one of the new RS4s myself, but sense prevailed - as I've only just managed to escape from the grip of oily engines, skinned knuckles, late nights and complaints from the Wife[B)]
