Did you contact who did the retune?
Even with a hi-flo filter, you should be getting better than that! The 48 you were getting is about right for a stock B6. I wouldn't think you should suffer more than a couple mpg with a hi-flo filter and the carb work to match.
Heck, I get >160 miles before hitting reserve with my B12 w/jet kit and can (~39 mpg most of the time). Granted, my tank is bigger, but the extra liter is all in reserve capacity.