Originally Posted by Mike Benvo
+1 to what Roman said..
To elaborate however - the car is not going to advance timing past it's targeted limits at full throttle. So higher octane is most beneficial in extreme conditions (i.e, hot temperatures, high humidity, high altitude). In cold temperatures at sea level the car will hit its targets and higher octane won't do much, if anything. Then if you get a tune suited for the higher octane the car will pick up a lot of power.
There are also maps in the ECU which control the amount of timing pull that is allowed, etc.. In fact, there are tons of maps that are ignition related, it's basically one whole side of the DME.
Are your tunes setup to take advantage of higher octane? (like race fuel not 93) Or is this something that one would have to request?