the only reason you'll get more mpg/power with a webber is if the 2e2 isnt working properly. its a myth that they add power on a std car, but if you have one with modifications the webber has the advantage of being much eaier to rejet than a 2e2.
that plus repairing a 2e2 can be hard work, especially now that MK2s arnt so common down the scrap yards, back when I had mine they used to get a couple new ones every fortnight or so, so i had my pick of spares. Once I got it going it was very good, used to get near 300 miles to £20 thanks to the 4+e box it had.