I have been fighting with a Makita orbital sander and decided to get an adapter to connect it to my Ryobi Shop Vac. It didn't quite fit, so duck tape came to the rescue. I started sanding plaster top coat on a wall and was astounded at how well it worked. It wasn't dustless, but it was far less than usual. So, now it makes me wonder how well a purpose-built system would work, particularly Festool. I've heard that Festool does dust collection better than anyone, so anyone having used a similar sander and now has a Festool can tell me if the Festool would be equal or better to my jerry-rigged Makita, shop vac, and duck tape.

I’m sanding wood not plaster so not sure the difference. With a 6” random orbit sander and the festool vac I see no dust on the surface or floating around my work surface at all. If I wipe my hand across the surface there is a small amount of residue on my skin, but it is not airborn.

Any brand of shop vac can do the job of dust collecting. The differences mainly lie in convenience (auto start. e.g.), noise level and filtration. I use a hepa filter for my Ridgid - https://www.homedepot.ca/product/ridgid-5-layer-hepa-pleated-paper-vacuum-filter-for-189l-5-gal-ridgid-wetdry-shop-vacuums/1000103772?eid=PS_GOOGLE_D00%20-%20E-Comm_GGL_Shopping_PLA_EN_All%20Products_All%20Products__PRODUCT_GROUP_pla-1769624053882&gclid=EAIaIQobChMIyc2U2-GQ_wIVzwCtBh0EtwBVEAQYBSABEgJHD_D_BwE&gclsrc=aw.ds

In terms of effectiveness, I see no difference between my Ridgid (hepa) or Craftsman (hepa) and CT15.

I agree that any shop vac will work, but dust collection works best then there is a high volume of air moved, not a high suction.

For example, a $300.00, 1 hp Grizzly unit is rated at 500 cfm.

By comparison a $170.00, 14 gallon, 6hp Ridgid moves about 179 cfm.

While I could find cfm ratings and suction ratings for some shop vacs, I could only find cfm ratings for dust collection systems.

From everything I’ve read, for dust collection, cfm rating is the important number.

I believe that depends on the type of shavings being created and what is creating them. For instance a dust extractor (vacuum) with high velocity and low volume is a poor choice for collecting shavings from a thickness planer. Just like the dust collector with low velocity and high volume is a poor choice working with a palm sander. I believe our British friends would say, "Horses for courses". If the machine creating the mess is helping by throwing the waste in somewhat consistent path, I think an extractor is generally the best choice. If the mess is uncontained, seems like a dust collector stands a better chance. All that said, I usually let the machine decide. If it has a smallish port, I figure the manufacturer knows an extractor is in order. If it has a larger port I use a dust collector.

I have been fighting with a Makita orbital sander and decided to get an adapter to connect it to my Ryobi Shop Vac. It didn't quite fit, so duck tape came to the rescue. I started sanding plaster top coat on a wall and was astounded at how well it worked. It wasn't dustless, but it was far less than usual. So, now it makes me wonder how well a purpose-built system would work, particularly Festool. I've heard that Festool does dust collection better than anyone, so anyone having used a similar sander and now has a Festool can tell me if the Festool would be equal or better to my jerry-rigged Makita, shop vac, and duck tape.

Funny you should ask. I got into buying Festool sanders and the Dust Extractors exactly for Plaster work back in 2004, not woodworking as a 1st choice for spending a fair bit of money at the time.[ ETS 150/5 and a CT-22]
 The fact that the same sanders and Vacs worked as well for actually woodworking once I need them to, was just a bonus.
 What really helps for Drywall Compounds, or Plaster work, for me, is having a Cyclone set-up before the Vac, so I don't fill the Vacuum bag, or its all important filter, with fine Drywall compound from sanding.  Much more so than woodworking dust in my experience.  Early on, before I had Festool Sanders, I killed one or two regular Shop Vac type vacuums, trying to use them for Plaster Dust Collection, hooked to a Bosch Sander I had at the time. Different animals compared to a Festool or Fein Vacuum, so I did more research before finally taking the Festool Plunge.
  If you only do occasional Plaster work, you won't need to get a small Cyclone. Just have fresh bags on hand for your Festool Vac. And keep an eye on the Vac filter often.
  I also have the Cyclone mounted on my CT-33 Auto-Clean Vacuum, so its a Festool Vacuum that was designed to help self-clean with sanding Drywall type dust in the first place. If you DO have a lot of work with Drywall or Plaster, this IS the type vacuum you'd probably want from Festool in the first place.  I bought mine used, they do come up for sale as someone in the Drywall or Plaster Trade sells off tools in a bad year, or changing out to another Brand/tool set-up.
 With this set-up, I can sand floors with a Conventional Floor 8"Drum Sander, fill up the Cyclone box with dust, and not the Festool Vacuum. Anti-static grounding has been an issue for some users with a 3rd party Cyclone attached to a Festool Vacuum, but not for me, with a Festool AS 50mm hose between the two items, and grounding straps/strips on the Cyclone box as well. Regular AS hoses from Festool for all Cyclone to tool extraction as well.