That makes sense. I have a Z18 and it is definitely enclosed in a glass or acrylic box so temperature changes are reduced but not eliminated. I had similar problems with my original Thing-O-Matic, I used sheets of plastic (acetate sheets used for overhead projectors) to enclose the sides but kept the top open just like you have done. I also added an aluminum plate on top of the heated build plate to have more thermal stability. Made a world of difference on that printer when printing ABS, I still use it occasionally for small stuff. After digging around in the custom print settings dialog of the latest Makerbot Desktop app I confirmed there is a way of setting the build chamber temperature on the Z18, if what you've said about temperatures above 67° causes problems I'll probably never use it.
Yes, I just received my first Smart Extruder+ and it's solved issues where I had misaligned layers and delimitation on extreme overhangs. It's working very well for us, far better than our first several months with the original extruder in the fall of 2014. I have about 200 hours on it and it hasn't missed a beat, longest print was for 27 hours and it auto-paused when the filament ran out. I loaded a new roll and it picked up where it left off, the only thing I could tell was a very slight bump between rolls on the print. Now that things have stabilized I'm looking for larger projects to use it with, for what I've printed on the Z18 a Replicator could have handled just as easily.