I posted this elsewhere but thought it could be of interest here as well. As a side note, I've been riding this for 3 weeks. I'm in the US, but bought it from a shop in the UK which shipped it to me.
Wanted to post about my experience with the new Ultegra RX rear Derailleur. I installed it on my gravel bike, an Ibis Hakka MX. The rear cassette is an XTR 11-40 unit. The derailleur isn't rated to that cassette size but it works perfectly well, the b screw is only screwed in about 2/3 of the way. The shifting is precise, accurate and works as you'd expect it to on a smaller road based cassette. This is particularly important because I'm running it 1X.
Regarding the clutch action, Shimano says that the tension is (slightly) less than on an MTB unit. While I'm sure this is true, it doesn't "feel" true when comparing to my mountain bike clutched derailleurs. They feel equally strong. When the clutch is off, the unit works like a regular road derailleur. Even when the clutch is off there's a fair amount of tension in the spring. I havn't ridden on exceptionally rough roads or trails but on regular gravel whatever that is, the derailleur is pretty stout. For example bunny hopping doesn't make the chain slap, even with the clutch off, however, the Hakka MX has a dropped driveside chainstay. Your mileage may vary in this regard.
With the clutch on, the bike over gravel and bunny hopping or impacts is a lot quieter. Not because it's necessarily keeping the chain from slapping but because it's eliminating erroneous chain movement. In the workstand there is definitely a little more driveline friction when the clutch is on particularly in larger cogs. However, on the road it's not noticeable at all really. On that note, on the road, I almost never use anything but the smallest 5 cogs of the XTR cassette (42T up front), and in those cogs the added drag of the clutch is less and generally the chainline is better. Shift effort in the stand feels slightly higher; out on the road, it's impercetible. In fact, you can shift (mechanical) to a larger cog with one finger, at least I can. It feels no different to me. Shift precision remains the same.
I basically leave the clutch on all the time, even on the road. Here's why, in my experience, even when running a lightweight XTR cassette, there is still enough inertia that when cranking really hard and then coasting the chain slackens significantly. So much so that if the Hakka MX did not have a dropped stay it would easily hit the stay every time. This never happens with road style cassettes or 2X. But for me, running 1X with a big cassette, it happens. When the clutch is on, it totally and completely eliminates this inertia induced movement. So, even on the road I run it with the clutch on.
I bought it retail for $115, it weighs 242 grams on my scale and in my viewpoint is well worth the security.