well we've already stated that your gas mileage will suffer. You've stated you run your car in open loop (cause thats smart). So yes your already slow car will probably be slower.

If your car isnt stock, you should tune for your mods to get the most bang for your buck. You arent going to find straight gasoline around here, so tune for E10, solve all your problems with one tune. that would be the smart thing to do.