I have sanded drywall before with my bosch sander and CT22, it worked very well, but I didn't have to sand much. (I only recently got my rotex, and I haven't tried it on drywall yet.)
Drywall dust does tend to clog the filterbags way before they're filled up. It's similar to the dust from grinding bricks, mortar or concrete (but grinding throws the dust way faster, so you notice it quicker when suction decreases) . When you have the collect that sort of dust, it is better to use a vac with automatic filter-cleaning. (like the vac that's meant for the planex) Festool makes an accessory for the CT that holds the planex, but they recommend it only for smaller jobs.
I have a wallsaw with both watercooling and dustcollection possibilities, when it wasn't possible to use the watercooling (risk of waterdamage) I would use my CT22 with it, and it would get most of the dust in the beginning of the cut, but suction would soon lower dramatically. And because we also have a wallchaser and a dusthood for a 230 mm angle grinder I went looking for a vac with electromagnetic filtercleaning.
When I was collecting information on those vacuums to see which one would be the best choice for me, I noticed that the vacs of the different brands showed significant similarities. I think the vacs with automatic filtercleaning sold by Bosch, Spit, Milwaukee, Hitachi, Metabo,... (there might be others) are made for them by
Starmix And those vacs sold under the Makita, Protool (->festool) and similar are made for them by
Nilfisk-Alto
I was seriously leaning towards getting the protool, because it uses the same hoses as festool (only in orange), but I had a few questions that my dealer couldn't answer (like the difference between the push to clean version that was just over 300? and another version that was about 600 ?. They also had a Makita, similar to the expensive protool, but it costed only slightly less, and uses different hoses; and a Spit which was 500?, but they had a spit-paslode-rep in the shop at the time who could explain the benefits of the vac (polyester vs paper filters and some others), and when he lowered the price to 375 I got that one.
It works well, I just have to remember to turn the tool I'm running from it off every once in a while, because the filtercleaning only works when the vac isn't running, and when the filter does get clogged in the meantime, the vac shuts itself off, without cutting the power to the tool. The spit came with a very long 36mm antistatic hose, which lockt onto vac, but the festool hose fit perfectly too. The spit works without bags and the fine but heavy dust from grinding and plasterdust is easily shaken from the filter, but I have to be careful it isn't hooked up to a saw or planer, because it doesn't work with wood chips and dust. I'm thinkin of getting bags for it so I have an extra vac for the kapex.
Sorry for the long reply, I'm tired and tend to get lengthy, I hope it is coherent enough.