Resurrecting this thread because a) Cyclingtips did some testing that attempts to answer the general question if clutch RDs add drivetrain friction and b) I actually got a gravel bike with Ultegra Di2, and I got a deal on an Ultegra RX Di2 RD.
First,
Cylingtips did some testing with Friction Facts, video version of the article
here. Basically, Shimano clutch RDs seem to add about 3W of friction when you downshift to an easier gear, but not when you upshift. The issue is that their clutch gets partly loaded and doesn't reset itself. While partly loaded, it adds that 3W of friction. SRAM's RDs don't perform in this way, and they added no friction in either direction in testing.
The video chalked this up to an issue of manufacturing tolerances. Also, both the video and the article explain that your clutch can get reset if you downshift enough (about 5-6 teeth total) or if you hit enough of a bump. If you are on gravel or cobbles, then chances are that you'll hit such a bump fairly soon.
The article also seemed to suggest that the ideal setup has a high clutch tension but a low derailleur spring tension. You can adjust the clutch tension easily on an Ultegra RX RD. If you try this, do scroll down to the comment in
this article by jamesb and the reply from Il Falcone. Jamesb said that his RX Di2 R was struggling to shift to larger cogs. Il Falcone suggested his shop had seen similar experience, that it could be corrected by reducing the clutch tension from the stock setting, and that the Ultegra RX Di2 RD's motor wasn't strong enough to shift with increased spring tension.
This is not actually my experience. When I put on my RX Di2 RD, I increased the clutch tension to near maximum. It does seem a bit slower to downshift. However, in a recent gravel event, it was able to downshift to all cogs, even in the big chainring. The guy I bought it from on the Paceline forum said that the motor wasn't strong enough for his 1x setup with a ~40t or larger max cog. I have a 2x setup with a 34t max.
I could not notice an increase in friction overall when I installed the RX RD. This probably just means I'm not very sensitive. I didn't do a full gravel event on the original Ultegra Di2 RD.
The original post is about an RX RD on a 2x road bike. The OP is likely to lose some watts in the scenario I specified, and I'm not sure I can see a strong rationale for this on most road bikes. Having my chain not bounce around was helpful on a gravel bike. On a road bike, you'd have to mostly be riding on cobbles to have the costs outweight the benefits, in my opinion.
