I've made some mods to my 1.9 L exhaust - I took the Y pipe and cut it open to eliminate the huge restriction where the pipes are folded down to "D" shapes - that's gone and cleaned up. I put in a huge magnaflow cat and took a 12" x 1/2" drill and drilled out the center of the exhaust. With an auto trans I went back to the same pump I filled it up with this morning - put 173 miles on it and got 18.4 MPG at 64-67 MPH on the highway with some 45 MPH driving (about 10 miles). I used to get 15-16MPG. http://www.pbase.com/spacecommander/westfalia_vanagon_mods
see how I eliminated the exhaust restriction.  It's a little bit louder now with the exhaust mod.
I can tell when the tank is full - it spills out. I think putting a 2.1L exhaust on a 1.9L would be good for 1MPG and a few horses as they don't have this horrible J pipe.

Tom
"Spiny Norman" '84 Westy
"Disnsale" '86 Alfa Spider