Anyone try this? I need to get an XT derailleur to fit an 11-42 on my bike with Shimano STI's.

https://www.wolftoothcomponents.com/products/tanpan

It works... as well as can be expected. I'm currently using the 10-speed Tanpan with Ultegra 6700 STis, a 10-speed XT RD, and a Sunrace 11-40 cassette. Shifting at the bottom end of the cassette-- notably the 11T and the 13T-- is not perfect, nor would I expect it to be. Sometimes when going from the 11T to the 13T, I have to up and down shift twice to get the chain to move-- I suspect this is just the nature of pairing a lightweight road shifter with a burly mountain RD. The XT w/ clutch requires a whole lot more force to swing than any typical short or mid-cage road derailleur. But the Tanpan does hold the chain in the selected gear just fine. I have no issues with chain rub in any gear, and it stays in the big 40T like it should.

I don't know if you have 10-speed or 11-speed, but if you have 10, you will also need a Goatlink to clear the big cog on the cassette. The XT RD will go up to 36T without issues, but needs the Goatlink to fit a 40T or 42T.

Why Shimano sees fit to have a different pull ratio for everything, no one will ever be able to adequately explain.

Thanks for the feedback. I'm using R685 shifters with a 6800 GS. It looks like I can just use the Ultegra derailleur with Roadlink up to 40 tooth. but if I want to step up to 42, I'd need to get the mtb derailleur paired with the tanpan. About a $100 difference for 2 extra teeth. Hmm...

Anyone here using a Roadlink with a 40t?

Tried it. The Ultegra derailleur simply doesn't have enough spring tension. Can't back-pedal at either end of the cassette or the chain just drops (back-pedaling with the chain on the 40T will drop down 4-5 gears,) and the chain slap in the 11T is nothing short of epic. Ultegra RD + Roadlink will manage a 36T fairly well, but the 40T is a no-go in my experience.

Just setup my bike with a road link and 40t cog.

Works great. The chain will drop to the next lowest cog if you back pedal in the lowest two gears, but I don't see that being a major issue.

Setup was a bit more finicky but it shifts like my previous 11/34 and 11/28 did.

The chainline is pretty whack on the bottom half of my cassette but I'll be spending 90 percent of my time in the top half anyway.