Hey 4444 welcome to the forum
1) are they really that efficient? what is the real mpg they get?
-Most likely between 60 and 70 MPG stock bike single rider
2) how reliable are they?
-The new UCE engines are very reliable, that said there is no motorcycle manufactured that has every bike turned out trouble free
3) is there any way to carry around about 6000 cubic inches of groceries on one via an attachment/attachments other than a side car?
-Sure there are a few different styles of smaller motorcycle trailers on the market, here's just one.
4) is there any way to get a larger gas tank?
-There's always a way to modify anything on just about any motorcycle if you have the know how or money
5) and will this air cooled engine do poorly in 100 degree plus weather? -6 degree or worse weather?
-They are the work horses of India where it regularly gets over 100 degrees, I can verify that as it's been that hot here in Pune the past few weeks and just getting hotter. And the UCE engines in particular were designed to operate up in the Himelayas so the cold and high altitudes are both good to go.
6) is there any way to attach a full wind shield and or knee shield? (i'm worried about people tossing crap out their cars)
-Yea there are many different types/styles of windscreens to choose from and as for knee shields you can either add engine guards and make/buy a cover or look at India based accessory vendors, I've seen leg /knee shields on afew RE's here so I'm sure they are available.