I don't get terrible milage. My bike is a Mo' Better Ported EG225 (1995 200 base) with a ProCircuit works pipe and MT Delta-2 V-Force reed cage. Gearing is 13/48. 1600' elevation. Crisp, clean jetting in the 35mm stock carb.
As always, milage varies with conditions, jetting, blah, blah, blah....
But, doing alot of high speed riding on dirt roads and railroad grades (lots of WOT and hard acceleration) I can get around 60 miles or so without running the tank dry. I'm pretty sure it was a little better before the big bore, but to tell the truth I don't remember what it was - I don't think I ever really checked it. From a different perspective, I can ride almost all day now (6-8 hours) on around 6 gallons. - At least the distant fuzzy memories of summer rides gone by tell me so.
If you don't need to ride extreme distances, go for the big bore. It's very worth it!