Well the thing with this card is that its a very highly clocked version and as such draws more power than a standard design. Obvious really.
Zotac have used the same cooler on the Amp version as the standard version, so what you end up with is a highly tuned chip running on a standard cooler. Not good in my book.
Nvidia say 99c is max.
90c is about right for the card. As you have already uprated the case cooling the obvious other option is to upgrade the cards cooler to a decent third party cooler.