How much does that run you? I can't seem to find any unleaded 100+ octane for anything less than $65, for a mere 5 galons.Also running up to an E30 is actually pretty safe as well. I limit mine to E25 to get me to a 93-94 octane as I run a couple of gallons of VP100 Unleaded per tank as well.
On a semi related note, it's funny how different gas stations give different results. I have a shell and mobil near me. The mobil 93 is definitely 10% E and is roughly 40 cents cheaper than shell, but the car acts weird on it. Now if I feel like taking a 20 minute trip to a different shell station that has the same price of the mobil, I feel like I hit a jackpot. I noticed this over a year back but the car just feels so much better on whatever magic juice that comes out of those pumps.