I have the dust collector shrouds for both the fixed base and the plunge base. They are decently designed and work OK. The issue with using them with a Festool dust extractor is that the hose connection consists of two plastic tubes; one inside the other. This way different diameter dust collector hoses will fit one tube or the other. The inner tube's OD is a perfect fit for inside the tool end of a 27mm hose. The problem is that Festool tool end is too thick to fit between the inner and outer tube of the dewalt dust shroud so it only goes on about 1/4". I hope this makes sense. I realize it is tough to visualize. If you are willing to cut away the outer tube with a hack saw it would work perfectly.
I tend to use the plunge base or the fixed base without the shroud. This isn't because of the plunge feature but because the dust shroud on the fixed base router, even though it is clear plastic, totally blocks your view of the workpiece.
Hope this helps
Jay