I built up an EVO with Ultegra DI2, reynolds DV3K wheels FSA bars,stem and crank and I'm down to (I say down to because my original frame was warrantied and had 105 group )15.00 lbs. I've changed the seat, stem and looked at every part on my bike and I could still save a little on pedals , a new hollowgram crank but I'll replace those parts when I find a back up /winter frame and put all the old parts from my original bike on it.
I will say because I didn't need to replace my crank right away (have FSA carbon light) the DI2 only added 212 grams vs DA mech. group and I would do it again . DI2 is NOT a must have item but I have put on around 1,500 miles on mine and love it. It's worth the 1/2 pound..BTW evo frame at 1/2 price (due to warranty), i'm into it for under 5K