I dont think you will gain that much in switching from a good roadified MTB to a cx/touring commuter.
Most of the comparisons between speeds on MTBs and road bikes are comparing to a roadbike shod with narrow tyres, not suitable for the urban jungle. If you compare a bike using 26"x1.5" and 700cx28mm, then the differences are marginal. You can always fit an even narrower 1" to 1.25" if you need more speed.
A racing cx bike is very light but the frame is clean of all threads and brazons that a commuter needs. Lower-end cx/touring style bikes do have all the waterbottle/rack/fender fitting you may need but they are not ultra-light. You are right in thinking that a cx/tourer is more agile than a full expedition tourer.
If you have some cash to spend then you may find that upgrading your wheels is more cost effective.