Shell 91 octane should be fine for the car.
I'm using 93 octane exclusively and never saw what you describe.
Reason I asked is early on in this forum we had a few intermittent check engine lights comments based on gas used. Most were attributed to lower quality fuel while traveling or trying to run 89 octane instead.
I'm sure others on the forum have "gunned" their car without what you've described so must not be a common occurenece. I could see the ECU set to limit rev's or cut back on fuel supply ... but not the thump and pop, (backfire?).
If it occurs again, you should have you dealer check it out ... its not normal.
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