My .02 for a few posts in this thread.
To the OP, the gas milage difference is vastly due to gearing. The double OD in the t56 is the reason for all the 30+mpg v8 swaps. The same car and motor but with a t5 would probably get ~5mpg less.
At least some of the power gain between the carb'd vs efi ls motors is in the intake flow. Big turbo people (600hp+) going from the ls6 mani to a victor jr with an elbow and throttle body are gaining a lot of power, like 50hp+.
