It all depends on the quality of fuel and the horsepower level. It is sime physics that there is a difference between 91 and 93. For the guy who thinks "its negligible" why not run 89 or 87? If 91 is not noticeably different from 93 then the same concept applies to 91 to 89 and 89 to 87. In my case the difference between 91 and 93 is 2 or so degrees of timing in the the tune and an additional 3 degrees global because of detonation. 5 degrees of timing is pretty significant if you ask me but then some guys don't mind running on the ragged edge and tune their car to the limit...... just save some cash for a new motor, been there done that.
FYI- most 13s are running 560-585 stock in my area, you will see a few guys that recently posted numbers from CA that are in the 560s. Dynos mean nothing because a 700rwhp car that traps the same as the same car at 650rwhp tells the whole story.
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