I have the (older) Midi, the CT SYS and two old CT22s.
None is idea for sanding powder based fillers. The dust clogs the pores in the bags. The smaller the bag, the faster it clogs them. Even though the hoses on each machine is antistatic, the powder based filler becomes statically charged as it spins its way in to the bag, once there it clings to the lining of the bag and reduces suction.
At a guess, you will see the MIDI bag lose efficiency once it reaches about 15% full (if a new bag). If you are happy to lift the top off and tap the bag every 10 mins or so, you should be OK to about 50% capacity.
When sanding such fillers, I try to use my older, larger machines. They originally had paper bags that often split when sanding powder based filler. I eventually started to buy bags for the newer CT32 machines. The bags are cloth rather than paper and oversized. I have to use a hot melt glue gun to attach the old CT22 "nozzle/receiver" but being too big means that when I stop sanding there is a slight degree of the bag collapsing, which allows some of the dust to fall to the bottom of the bag.
The ideal Festool dust extractor is the expensive CTM35
www.toucantools.co.uk
In use, it whacks the bag to make the dust fall down from the top of the bag (which is pulled up towards the filter).
Bag clogging really annoys me. During lock down, I set up an in-line aluminium cyclone (to maintain the antistatic properties) but soon realised that it only captured heavy particles, not fine dust. I considered creating an additional water trap... then the pubs were allowed to reopen, and I decide to makeup for lost time.
I genuinely wished that I could say "yeah, the midi will be fine", but unless you are happy to replace the bag frequently or, at a minimum, to lift the lid and tap it, it will become a PITA. On the upside, I can confirm that the motor has overheat protection, as do all of the Festool dust extractors. It may cut out but 10+ minutes later it will continue to work.
Edit-----
I don't own the Planex. I don't work on tape and join plasterboard. I do however have 90mm, 125mm, 150mm and 180mm sanders, plus the old DX93 delta, the RTS400 and linear LS130. If you are sanding plastered walls, and don't mind using a hop up platform. the 150mm Rotex would be my go to. If you are sanding tape and join ceilings, then yeah, the Planex (which from memory is 225mm).
As an aside, the CT SYS, whilst great for carrying on the tube, is barely able to deal with the dust generated by the Rotex150, regardless of what you are sanding. That said, the Rotex150, of all of the Festool sanders requires the most free airflow.
Edit 2- is your RTs 400 the new model or the older one? I have only ever purchased 2, the first one was seldom used and when I did start to use it one day it started to run, really, really slowly. I purchased a new one, which did the same thing after a while, under warranty, I had it repaired. Didn't use it for many months and the same unit did the same thing. I think the capacitor on their mini PCB is possibly the weak point. It is the only Festool product that I have been disappointed with.
