The locking dust port does fill up with drywall compound dust. But, if you leave the sander/vacuum on and slowly disconnect the hose from the sander, the hose will capture the dust. No need for a bucket or trash can underneath.
No, that really won't do as a practical solution - the whole point of having an integrated tool system (for which we pay handsomely) is to increase efficiency and effectiveness - you plug the tool into the power outlet on the vac so that when you operate the tool the vac turns on and off automatically. The solution suggested here is that you go back to the vac, turn it on manually, disconnect the hose (with a complex motion to catch all the trapped dust - I've tried this), then return to the vac to turn it off again manually.
I am one of the biggest Festool junkies there is, but IMHO this new vac connector is simply a flawed design and has just caused unnecessary problems.
Re the new vs old DTS/RTS 400's, overall, I think it is fairly balanced, so I would be happy with either (or both in my case!).