Ok, so the above is one perspective and worth considering, but comparing a Shimano mech group- 10 years removed - to 2013 Red is somewhat apples to potatoes when contemplating a move from a current Shimano electronic group. Although undoubtedly well-maintained, the 7800 group in question is old tech two generations removed from what both Shimano and SRAM are offering, and that's only considering the mech side of the equation.
First, yeah, I agree with everyone who said "go ride Red." Preferably, bring your Di2 rig and do extended back-to-back rides. If you still don't absolutely have to own Red at that point, don't because you have other options.
Given that the OP expressed no specific dissatisfaction with his current Di2's operation but values weight reduction, I'd suggest that maybe groupset isn't the best upgrade for his build goals. Not only is the difference in mass almost completely imperceptible on the road, there may be other upgrades (e.g., *Nice* wheels and tires, latex tubes, cockpit) that might offer both the weight reduction he seeks and more gratification on the road.
Consider these opportunity costs when making the decision. I'm not sure what wheels you use, but there's an excellent chance that you could obtain a huge upgrade or a procure a new type of wheel (climbing, aero, tubeless, roubaix-style tubular, etc.) to fill out your collection and use on all of your bikes.
1) Are you comfortable spending more time and money on the bar taping, cable cutting, and derailleur tuning that will become mandatory with this switch? Time wrenching is time spent not riding. This is a big reason I went with Di2.
2) Are you going to install this and maintain it yourself? Experience with the Yaw derailleur's function seems to run the gamut from "no difference" to "revelation," but I've seen skill levels from pro mechanic to amateur express varying levels of frustration setting up the system.
3.) I can't over-emphasize how different Red double-tap and Shimano Di2 shifters feel and function. Along with function tests, this is the #1 reason that a test ride should be your #1 priority if you continue to pursue this option.
4.) Seriously cross-reference the brake function. Even Ultegra-level brakes are reference points for ease of set-up, modulation, and absolute power. Brakes are one area where it never pays to favor weight over function. Red 2013 scraps the dual pivot, bearing-borne brake for a single pivot, cam actuated, bushing mounted brake that (at least on paper) looks a lot like the controversial Zero-G. I'm sure the SRAM setup is less temperamental than the Ciamillo system, but that would be an area of intense interest if were evaluating this new group against Shimano. Having not ridden Red 2013, I know this is one of the first features I will test when I get a chance to ride it.
5.) Please report back and tell us what you think
Honest and eloquent real-world ride impressions (and comparisons) of current equipment are worth 1,000 posts of speculation, mine included! It's always good to have real rider impressions for others to reference via search.
Enjoy the test, and do explore the wheel options if build weight is your foremost consideration.