The gas mileage conversation is one that we love around here... So the answer to your question is maybe. First off for reference my bone stock well worn 350 with grocery getter gears and a 2bbl carb gets between 10 and 12 mpg around town. Not super far off but you should be doing better even with bigger cubes and enhancements. Some guys claim horrible gas mileage and others seem to rival a Prius.

My question would be how is it running? Do things seem good? Is it running rich? Idle good? In other words if you never measured the gas mileage would you have reason for concern?