It will likely increase performance and fuel economy. The 1st 5-6 gears or so will be much more tightly geared and more aggressive for less rpm drop and better gear multiplication.
Look at the Chrysler 300 V6 for example. The 5 speed car did 0-60 in 7.3 while the 8 speed car does 0-60 in 6.6 despite being 391 pounds heavier than the previous car (not tranny, overall weight). Combine that with 31mpg on the highway (6 better than the 5 speed) and thats a great combo.
Source C&D
5speed vs 8speed 300 V6.
Here are Chryslers gear ratios. I suspect GM/Ford will be similar but have some unique gears.
Gear 8R70 8 speed
1 4.71
2 3.14
3 2.10
4 1.67
5 1.29
6 1.00
7 0.84
8 0.67
Gear WA580 5 speed
1 3.59
2 2.19
3 1.41
4 1.00
5 0.83