I'd get an XTR RD.

I'm on the new Rx805.

ultegra RX should be available soon

Been out a month now. Just a pain to find

Yup, XTR or XT is the only way to go if you want electronic shifting. I think both work with road or TT shifters, but I'm not certain. OP: you should clarify if you're looking for 1x Di2 or mechanical. Road 1x Di2 isn't technically offered by Shimano at this point.

RX is available as Di2 or mechanical

Both. I've had mine a few weeks now I got the di2 which is the RX 805. I submitted an article to Dan about it he hasn't posted it yet

notice more strain on the battery? that's the claim. its probably overkill for me but would be nice peace of mind, and I'm a gear nerd.

Oh. Didn't know that. There's your answer.

Nothing noticeable yet but I've only had it a few weeks and then I got sick and I've been off the bike about a week so hard to say

Sorry first time I read your post I misread thought it was a question.

RX805 only specs a max cog of 34T though. I think if he wants to do 40 he needs to XT/XTR.

That is correct

For a Tri bike, I would avoid all the Shimano clutch derailleurs. (1) The clutch provides zero benefit for a non-gravel, non-MTB bike, and (2) the clutch introduces a few watts of drag to the drive train.

I would go with a long-cage (non-clutch) Ultegra Di2 or Dura Ace Di2 rear derailleur. They shift as fast as you want to program them to shift -- and they shift flawlessly. They probably won't get you all the way to 11-40T cassettes, though.

The loss of Watts due to drag is primarily seen in the mechanical version of the RX Shimano derailleur. And I've tested that on my trainer with a power meter. there is little to no difference with the di2 version of the RX 805 when it comes to a loss of power due to the clutch.

I respectfully disagree. I ran XTR Di2 rear derailleurs for two years on my gravel racing bike. The drag is annoyingly ever-present with these derailleurs. It's not a HUGE number, but it's there. The drag eventually drove me to use a non-clutch derailleur even on my gravel bike. (I still never threw a chain.)

I'm unfamiliar with that rear derailleur so I cannot agree or disagree with you but I will take your word for it. The only thing I can say is I have the RX 805 in the di2 and I've tested it and there was nothing that didn't fall within normal watt variation from one ride to the Next.

Wouldn't you need 2 power meters for this?

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You mean the test one derailleur no you would not. the other nice feature of the RX rear derailleur is it has an on-off switch so you can in fact turn off the clutch if you're concerned about that very thing. I tested it with the clutch on and with the clutch turned off both sets of numbers fell within range of what the same bike tested with a 9070 rear derailleur

I'm not in the know on this stuff, but I thought you could turn the clutch off? I installed XT 1x on my old MTB last year and I think it has a switch for the clutch. Also, how about a chain keeper on the front ring? I mangled an old front derailleur for that purpose, but there are more elegant solutions.