Ok , need to step back

I wouldn't worry about the gas mileage, how much worse can the 470 be?

Say you drive a lot and put 5000 miles a summer on. and the 340 gets 15 mpg and the 470 is terrible and gets 7.5 mpg and not sure what your gas price but say it's $2.50, 340 cost $833 a year 470 $1,666 a year, That's if you put on that many miles on and the 470 gets half, which it should be better than that unless it's poorly setup or really radical, and the cost of the swap probable eat up a year or two savings anyways. I find when doing more then a good tune it's cheaper to put the money in the tank then trying to gain mileage from upgrades usually not cost effective.