on '05 to '09 RS/2.5i (probably many older and newer models also) 87, 89, 91 and 93 octane gas will still not have the anti-detonation properties to avoid timing advance from being reduced (IAM < 1 (16 on 16-bit ECU)) 93oct will allow the most timing advance to be used. for example, on a 75F day 93oct resulted in an IAM of 0.9xx. 87oct resulted in an IAM of 0.675. This data is true for my '07 with the factory EL manifold and factory exhaust with hybrid intake and stock tune. The colder the intake air temp the more timing advance can be used typically. without an IAM of 1.000 (16 on 16-bit ECU) the ECU has determined that knock has occurred in at least one load/engine speed area and it has removed timing advance to eliminate the knock. this means that there is less tq being made at that load/rpm than could be. It also makes it more likely that knock may occur at other load/rpm's. BUT, the ECU should do its job well enough to keep detonation under control for the typical daily driver. It has to knock initially for the ECU to be able to recognize that timing advance needs to be reduced, if it in fact does need to be reduced.
miliage will increase and more tq will be made anytime more timing advance can be used. So higher octane gas will result in a little bit more power and slightly better miliage.
Blame the aggressive factory tune for this. It does maximize the potential of the gas being used at the cost of an increased risk to knock.