Got a Shimano rx100 7 speed derailleur. What’s a good option for upgrade or is it leaving it worth it? I’d also like to ditch the down tube shifters and upgrade to brifters. Thoughts, recommendations? Cheers!

That’s solid equipment, will happily interface with Shimano 8-9-10 speed shifters of the STI variety.
Just change the shifter, cassette and chain and adjust the RD, you’ll be fine.
Feel free to change it if you must, but ot at all necessary.

I have read many times that RX100 components are the same as 105 with a different finish. May/may not be true.

Vintage Dales are great candidates for neoretro builds (especially if the rear spacing is 128mm and not 126mm). Lots of choice out there for brifters, kinda depends on your budget. But if I were to replace the RX100 groupset, it certainly wouldn't be for a performance wise pretty similar 8 speed groupset with the early brifters.

Sounds like a case of upgrade-itis to me. If stuff is broken or worn out, then upgrading makes sense. If it is still working OK, "upgrading" doesn't make sense to me. Certainly not with the derailer. Maybe replacing the downtube shifters with integrated levers, but only if you are having trouble making the shifts or are reluctant to shift with the DTs.

You could "upgrade" to Ultegra, but you would notice any difference. Spend yr $$$ elsewhere

I rebuilt my old (1994) Cannondale R500, with full 8 speed RX100. I went for DT shifters, but there are 2x8 RX100 STIs out there.

Cheapest option is to just pick up a set of new Shimano Tourney STI levers and stay 7 speed.

I'd leave the current derailleur, unless you're a weight weenie. An upgrade to same era 105SC would save 57g, while 600 Ultegra would save 65g. You shouldn't notice any difference in shifting performance, unless there is something wrong with the derailleur itself. If you want to keep everything period correct there was a 7 speed Shimano RSX brifter during this era.

Yep. Though those RSX brifters are quite pricey. Glad I have one in the stash.

I also think, depending on the spacing (if it is 126 or 130), if the OP wishes to upgrade to brifters/8 speed, there are reasonably priced options to do so. Chain/cassettes are pretty reasonable, and 8 speed brifters are actually less pricey at times than 7 speed.

What kind of bike are we thinking about spending money to upgrade? That’s a key to the correct answer.

The 7 speed Sora ST3300 is not quite 'period correct' but it does look the part with the silver levers.
Otherwise you could get brand new Tourney brifters or Microshift R7. Unfortunately these newer brifters are all black.

By the way, if you have the RX100 FD-A550 front derailleur, that needs to be replaced by FD-A551 to function properly with brifters.

rx100 is solid and and underated.

Absolutely agree. But I guess I assumed the OP wanted to transition to 8 speed or at least brifters? I could be completely wrong on that.

+1. RX100 on its own merits is good stuff. I think the only reasons to knock it might be the extra weight, and that the hubs use cruder "piston-ring" seals instead of the rubber seals used on higher-level hubs. But that's no issue as long as you repack the hubs when they need it.

My '91 Bianchi Eros came stock with an interesting mix of RX100 (brakes, levers, hubs, shifters) and Deore LX (crank, derailleurs.) Some time ago, I replaced the Deore parts with RX100 components to make it a completely RX100 road bike. Sometimes you just need a silly project.

Did you actually weigh the components to come up with this? I kind of doubt there is that much difference in weight. The 105 1055 and the RX100 A550 groups look really, really similar aside from finish.

If you do the upgrade, let me know if you have interest in selling the downtube shifters.
I need a set, I have a bike that was converted to stem shifters.

They look similar, but small differences add up. The RX100's pulley cage plates are both steel. I checked a 1055 RD and one is aluminum. And the 6401 RD has two aluminum cage plates.

It's also possible that the higher-tier RDs are more hollowed-out for weight savings, but I haven't checked that closely yet.

there are lots of sources for this data and Tmar is pretty much the expert. with shimano there is always some differences for groups in the same time, often weight.

He can do that and stay with RX100. But he might not really care about staying with RX100, that's really just an aesthetic choice. It's probably a lot easier to find 105SC or 600/Ultegra STIs and RDs. Me, I like RX100, so I built up a bike around it.

Will the RX100 index with 8 speed brifters as long as the cassette is compatible?

You have to get the correct generation of RX100, but yeah, A551 STIs should work with an A551 RD, and I THINK those are both compatible with 1055 (105) or 6400 (Ultegra) STIs and RD.

If you want to go triple, look for RSX brifters to go with it.. They were the spawn of 105 iirc.

I must have the wrong edition of this drivetrain - it is on a bike at a relative’s house I occasionally visit. Maybe it needs more use and maintenance. Stored in a dry garage.

Of course. The later RX100 groupset comes with 8 speed brifters. ie. the ST-A550.
As I mentioned earlier, you do need to swap out the FD-A550 in favor of a FD-A551 if you want to upgrade from the DT shifted RX100 groupset, to the brifter shifted RX100 groupset.