Well, if you can find rates for the R9 280x, and you know that the R9 280 has approximately .875 of the computing resources of the R9 280x, not taking clock rates into consideration, you can probably generate a pretty close estimate as to what your hash rates are going to be.
Furthermore, if you have been looking at graphics cards in the last three years, the R9 280x is the rebranded and slightly tweaked HD 7970, and the R9 280 is the rebranded and slightly tweaked HD 7950. You could always look for hash rates of the previous version of the card, knowing the new model will not be significantly faster, turning in similar results.
As for why you can't find hash rates listed for the R9 280 in very many places, it's probably due to the fact it hasn't been considered a high end card in a few years and that Bitcoin profits from mining on graphics cards has dried up. There are other currencies which can be mined, but there are likely few who want to try and keep up on the value of GPU mining for all of the knock-off currencies.