The second one seems correct to me, though I'm having trouble explaining why - even to myself. I think it's because we think of "10%" as a specific increment, a scoring unit, so we treat it as "please give me an additional kilogram of potatoes." Here, the unit is kg, which is countable, so we use "an" because we only want one unit.
You have to have an article (an or the) with additional unless it can be replaced by more, at least in BE.
An additional problem was that he could not speak Turkish. [you cannot say More problem]
Additional inconvenience was caused by the heavy rain.
Additional benefits include a swimming pool.
As Egmont says, it's an extra unit/quantity. We might say another 10% in some contexts.