I recently upgraded the Dura-Ace 7800 group on my Felt to 7900. The bike came with 105 from the factory, but I had a DA 7900 standard crank swapped out before taking delivery. The plan was to fill out the group with 7900 at some point, and last week was it. I'll try and compare and contrast the 7800 .vs 7900. To be fair, I bought the 7800 group used, so I can't say it was an apples to apples comparison. Take it FWIW. I purchased the 7900 group (shifters, derailleurs, brakes) on eBay from a seller in the Ukraine. Shipping was very fast considering all it had to go through. Total cost was $806 delivered.
The 7900 STI shifters have a very different shape than the 7800. The clamp end is fatter and more substantial, and the lever end is narrow, with a smaller knob at the end, and, of course, no cables sticking out of the sides. For the first few miles of my maiden voyage, I felt that I was stretched out too far, but it didn't take long to get used to it. I found that I have more hand position options on these hoods, both on the flats and on a grade. They are very comfortable once you get used to them. The extra reach puts me in a slightly more aero position with no stem change, and it further unloads the weight on my hands. There is a little less to grab onto when climbing out of the saddle, but the grip is comfortable with a nice sized notch for my thumbs. The levers splay outwards, which is very different than the 7800, and as I found out, that is because the lever throw is longer, no matter what Shimano says.
Silky smooth, positive, IMO as good as you can get in a mechanical setup, only eclipsed by DA Di2. With the KMC chain it's noisier than the 7800 but the sound is pleasant. Sweep upshifts are limited to 2 gears but that's not something I did very often when racing. It was a nice feature to use when training but I don't think I'll miss it much. Lever effort is a little higher than 7800 but that is to be expected with the hidden cables. One finger upshifts at the top of the lever are no problem. Lever throw is longer as I mentioned previously.
The trim function was removed as Shimano says that the 7900 front derailleur can handle ten cogs without it. I set up the FD in one pass and was not able to achieve that goal. I get a little chain rub at the extremes. This may be able to be tuned out. Downshifts are positive and lightning quick. However, I am having a little trouble with the upshifts. My left index and middle fingers were amputated in a childhood accident, so I shift the left lever with my ring finger only. Lever effort is significantly higher, at least on my bike and with my cable run, which may be able to be optimized. If I don't give it enough of a push, the derailleur shifts but the shifter does not click, which pops the chain back down on the little ring. I found that if I give it a double pump it works 90% of the time. I feel that this issue is caused by my own limitations and really says nothing about the product. I'll get it worked out.
The 7800 brakes were fantastic, but these raise the bar even higher. Excellent modulation, incredible power from the hoods or the drops, with the typical Dura-Ace adjustability. I cannot imagine anything better.