saddle: Arione 00 - 140g
headset: extralite, pick one of the 4 that matches - avg. 50g
handlebars: any of the carbons: deda supperleggera, fizik cyrano 00, 3T ergo-etc. LTD, Cinelli Neos, on and on - avg. 190g
those are a few, should save you about 300g for about $1000ish.
if you do warm up to changing the groupset, then well... the stock RED will probably save you 500g at least.
Groupset wise, I'm sticking to electronic, so, if I were to upgrade, I'd most likely go for Di2 9070. Thought about Campy EPS, but it seems that 9070 is both lighter and cheaper than Super Record EPS, so there's no point. And I can reuse existing wiring and batteries with 9070.
I tried to do the numbers for RED anyways, it looks like the savings are more like 350..400 g. FD: 108 g, RD: 125 g, battery: 70 g, shifters: 32 g, miscellaneous battery mounts and wires: ~90 g, offset by the need to run heavy shifter cables in place of light Di2 wires.
You could always sell the power meter and get a stages. surely the cost would not be too much in the change over ?
Stages power meter is not compatible with my current crank arms (it requires an alloy crank arm, there's a list of compatible cranks on the web site, and mine is not in there). It remains to be proven that I gain anything by selling the powertap, getting a lighter hub, getting a heavier crankset, and getting a stages meter.