the rest of my exhaust was free flow (3" pipe) no dholkis at all. Just an exhasut tip which had a slight muffling effect (and essential back pressure). Would not advise you to do the same as it will be too noisy to drive all day, every day. Instead just remove one and you'll have a good sound and manage to retain some back pressure.
After the work was all done my cars average fell to about 3, but not because of the conversion,... i just kept driving the bugger in the lowest possibel gear all the time (or rev the hell out of it).
If you drive the same as you did before the conversion you'll see that there is almost no difference in fuel consumption (some believe it actually improves)