A few updates. I'm now down to 6.410kg total.
I picked up a Berk Lupina on my recent trip to the UK: 150mm with round rails in 3K matte. It's listed at 80g, but mine came in at 70g, saving me 33g over my cheap Chinese carbon saddle (103g).
I also sold my Giant SLR 0 Aero (55mm) wheels a while back and have been riding the stock SLR 1 Climbing (30mm) wheels (1425g w/tape), which are great and a decent weight. I thought the lower profile looked better on the bike. My local shop gave me a nice discount on a set of the 2018 SLR 0 Climbing wheels, which saved me 66g. They're advertised at 1325g, but mine came in at 1359g with rim tape. Not a huge difference from the SLR 1, but I really like the quality and feel of the SLR 0 hubs. I'm keeping the SLR 1/Vittoria tires/Ultegra cassette setup for bad weather days.
Finally, I went with lighter tubes and tires on the new wheels: Specialized Turbo Cottons in 24c and Continental Supersonics. This saved me 103g over my previous Vittoria Corsa clinchers (500g/pair) with Continental Race Light tubes (148g/pair).
Here is my updated specs:
2017 Giant TCR Advanced SL2 (XS)
Ultegra 6800 levers, F&R derailleurs, brakes
Dura Ace 9000 pedals: 249g
Dura Ace 9000 crankset 52-36, 165mm: 631g
Dura Ace 9000 11-28 cassette: 195g
Giant Contact SLR Carbon handlebars (40cm): 181g
Giant Contact SLR Carbon stem (80mm): 122g
Giant Aircap expander: 15g and cut steerer tube (-55g)
Giant SLR 0 Climbing Wheels (30mm) w/rim tape: 1359g
Specialized Turbo Cotton tires (24C): 442g/pair
Continental Supersonic tubes: 103/pair
KCNC Titanium skewers: F 19g, R 21g
Garmin mount: 28g
Berk Lupina saddle, 150mm, round rails: 70g
Jenson Elite Carbon cages: 24g, 23g
Current weight with pedals, cages, and Garmin mount: 6.410 kg
I am now trying to decide whether to upgrade my Ultegra 6800 drivetrain to DA 9000, DA 9100, or 9150. And I'm shopping for some EE Brakes. I'm hoping these changes will get me down close to 6.0-6.1kg.