From most of what I read, 7900 is a little bit of a disappointment. Shimano may have rushed it as Sram has put a fire under their ass. My 7800 shifts and works flawlessly, but I prefer my Sram Rival hoods a lot more.
I don't see paying more for heavier 7900. I went with 09 Rival which is 100 grams heavier only than 7900 and $2k cheaper (retail price vs. my discounted price). Red would be the only group worth an upgrade for the weight savings but I hate the crank. Buy 09 Rival and with $ saved put a Vuma Quad crank on that same bike and you have both groups beat on weight.