what gears do you have currently? to gain 8 mph in 3rd i wouldn't even waste the time/money to swap gears. personally, i would just shift into 4th earlier and not wait until i topped out 3rd. id imagine your torque is almost identical in the top of 3rd/mid 4th anyway so the loss would be minimal on shifting earlier. unless you run solely 1 track you run into the issue in another gear at another track.
but in my current car i run a Jerico 4 spd with 2.94/1.91/1.72?/1 ratio with 3.08s which gives me a top speed of 168. running a 423whp motor
in the roush when i tracked that i ran a viper spec T56 with 2.66, 1.78, 1.30, 1.00, .74, .50 ratios and 3.73's 4th gear topped out at 135ish