You might want to consider/check other things that can rob your power, e.g.: the front rotors on my low mile Z (low 20s at the time) looked bad for the miles, so I decided to r & r them. On both sides, the lower slide bolts were rusted in solid! Obviously this caused drag, heat, and was probably the reason for the dogged looking rotors.
Popped off the brackets, shot them with penetrant, worked the bolts loose, buffed with the wire wheel. I noticed afterwards that I could coast for a lot longer distance after letting off the gas. Something like this could maybe cost a couple mpg/tenths in the 1/4.
Go to Synthentic oils though out the drivetrain. I use Mobil one though out but never test the threoy. Royal Purple did a study with all of thier syn. oil and gained 10 horsepower at the wheel. Good show I think it came on the dicovery channel a while back.
two of the options i can think of, is a trans with the internals that are completely micropolished or mikronited, and a set of mikronited rear gears.. That, combined with the good synthetic fluids should net you some gains.
I have a built trans thats completely micropolished and cryo'd, and im making the numbers in my sig (02) with essentially a lid, catback, and 3.73s.