If the numbers on the back panels of the units are correct then the answer is No, the 976 will not generate more heat than the 990. It will generate less.

But, there's always a but, the 976 MAY run warmer than the 990 because it's a smaller package. It's more about heat transfer than heat generation.

I would not expect the differences to be significant in your setup. Overall you are in a slightly better place from a heat generation point. If you have any moving air at all by the 976 then I think the heat transfer will be fine also.