I really want to hear the builds on these sub 20lb SF100, or other bikes. I read on here all the time the weight and sorry, I don't buy them. I have several weight weenie bikes and none are sub 20 except my Ti IF single speed with a lefty carbon. I have Medium SF100 with Token Carbon Seat, KCNC Seatpost, Richtey 4 Axis Stem/Bars, Stans Race Lights on DT 190 hubs, KCNC Rotors, Full XTR and I am just under 24lbs with an incredible accurate scale. I can drop another pound or so with lighter pedals, dropping to 1x and lighter stem/bars but that is about it. There is absolutely no way to possibly get it under 20lbs.
In my opinion, just compare frame weights to frame weights. You can build any of these machines with the same components but the frame weight is really where any real weight difference comes into play.
The Epic S-Works is just a tad bit lighter than the SF100 but good look getting one! I just canceled my order after placing an order in late July. The Tallboy is a great frame and I have rode one several times. But it isn't in the same class as the SF or the Epic.