there is an old (front page) article on here where tests were done on tubes (butyl and latex I believe) and it's effectiveness. It depended on brands, but latex tubes did fair better at sealing generally when sealant was added.

I suspect you would see rolling resistance act similarly to how it does in tubeless (more sealant generally = more rr)

A real downside is that when the sealant dries up, you don't have any way to remove it like you would in a tubeless setup.

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I've had moderate success in rescuing cx tubulars, with sealant.

Hit or miss with road clinchers and tubulars. Pressures above 40 psi tend to blow out anything larger than pinprick holes. I've often taken off blown latex tubes and observed tiny clusters and columns of latex where small holes have been sealed. I tend to use about 10 ml in a standard road tube - anything more and sidewalls sticking together tends to be a problem. Works a bit better for tubulars than clinchers.

I ran latex tubes with sealant for a few years. I had about a ~50% seal rate for sealing at a rideable pressure, e.g. ~40+ PSI. By comparison since I switched to pure tubeless that seal rate is ~90%.

I don't know for sure that different is just statistical anomaly or real. (though 2+ years gave me a good amount of flats).

But it was enough for me to switch to pure tubeless. I still sometimes run latex tubes, like in track or TTs on nice tarmac, where the risk/hassle balance for tubeless isn't great. But don't bother with sealant in the tubes anymore.

I'm a new convert to Stan's Sealant after using vittoria, Effetto & continental for years.
Stan's is in a league of its own; I'm reasonably convinced it would seal the Great Pit of Carkoon.

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There was a thread about this not too long ago. I've put Orange Seal into my latex tubes to stop slow leaks after pulling out thorns and it worked. But I don't put in sealant as a preventive ahead of time. My current latex tubes have no sealant in them.

Have no idea about the impact to rolling resistance. I definitely didn't feel any difference on the road.

This has been my experience as well. I had a front tube with 5 or 6 patches that was leaking above 70 psi so I put a bit of orange seal in. It does not leak anymore and the rate of deflation is around half of naked latex.

So this thread obviously jinxed me. I had another slow leak after removing a thorn from my rear tire post-ride and again used Orange Seal to fix it. The tire is nearing end of life but I didn't want to go through the trouble of removing it just yet.