My interpretation of this: "the extra damage is 2d8 for a 1st-level spell slot, plus 1d8 for each spell level higher than 1st, to a maximum of 5d8." Is that the maximum relates to the extra for spending higher level spell slots. In other words 2d8+5d8 maxiumum (from a 6th, 7th, 8th or 9th level...