Well, it seems you are not getting my point.
I've been sanding plaster all day every day the last year so I got some hand-on experience here. Gypsum dust clogs your dust bags very fast, like when it is only 1/4 full. So with an L or an M vac, this is going to be expensive very quickly.
Both L and M class vacs have the exact same suction level, and both will clog at the same moment, the only difference is the M class warns you when the suction level drops below an acceptable level and you know you have to replace the bag. In this regard the M class has the slight upper hand on the L class vac because with the L vac you have to find this out for yourself. Which, I might say, is not that difficult, you'll see when it gets messy pretty quickly.
I use a Festool Mini and a Midi vac for this, both are L class vacs. To mitigate the problem with premature clogging of the bags I have added a pre-separator (cyclone) which catches a lot of the dust and makes my bags last about 5 times longer.
By the way, it is not that important how much suction the vacs have compared to each other, because they all have more than enough to do the job. The problem is, if you have too much suction, your sander will get stuck to the wall like a suction cup, and this will negatively affect your work. This is why you often have to dial the suction level of the vac down.
Also, use the proper sander for this, some sanders are much better in collecting the dust than others. The Festool ETS series are great for drywall sanding and have great DC. I often use the ETS 125 for the smaller jobs, and it works almost dustless, but for the larger jobs I use the Rotex 150, and this one creates a lot more dust the vac won't catch. And then there are of course the dedicated drywall sanders by Festool, the Planex series.
I always wear a respirator when sanding plaster or drywall.