badceli, you increase the top speed by increasing the final drive ratio. For example if it's 4.312 you increase it to a higher gear of 3.0 for example. But that slows your acceleration so you'll hit the "top" speed later. Your theoretical top speed is fine with a final drive with 4.312 which is also the same in the stock GT tranny. With the extra power you'll be hitting higher speeds sooner of course. But as far as the "top" speed you want I don't know. What speed are you thinking of?
According to this website: http://www.f-body.org/gears/
your theoretical top speed in 6th gear is 173 mph redlining at 7000 rpm (and with 205/50/17 tires). That 6-speed is the C2 tranny for the GT.
With a Final Drive of 3.0 you can theoretically hit 249 mph in 6th, with all the same stuff mentioned as above.
But if you don't have the power, not only will you get up slow, you might not have the power to hit that theoretical gearing speed due to wind resistance and rolling resistance.
Going downhill helps yes, and so would a wind at your back as opposed to the other way, but of course there's still air resistance.
Personally I wouldn't change the final drive ratio unless you make around 650 hp.
Why does the McLaren F1 which has 680 hp and has a top recorded speed of 240.3 mph only do the quarter mile in 11.5 s? The answer is that it has a final drive ratio of 2.37.