I'm no good with BSTs or any of the technical stuff for that matter. I'll stick with my sorta defined role for trying to create visual aspects of pokemon. :(
Sorry I suck.
Anyone can be a BST wonk given a short amount of time. Art (at least good art) may be a skill, but it takes years to develop it well for many people. Especially internet Art since mice aren't exactly as sensitive as the human hand.
Advantages of 85 Speed:
Can outspeed Max Speed Jolly Breloom (269 vs. 262) without +Speed nature.
Ties Heracross.
Allows a pokemon to beat Max speed Base 130's with Choice Scarf without a +Speed nature.
Disadvantages:
There are loads of 80 Base Speed pokemon that use +Speed natures.
Still rather middling.
Takes away more points from other stats.
Advantages of 80 Base speed:
Outspeeds nuetral speed natured Heatran, Metagross, and Breloom.
Beats everything below Base 130 speed with Choice Scarf.
Disadvantages:
Needs +Speed nature to beat Base 130's with Choice Scarf.
Shares a large number of pokemon with +Speed natures.
Advantages of 75 Base Speed:
Beats nuetral natured Breloom and Heracross.
Disadvantages:
Very middling, and slower than a lot of base 80 and 85s.
Advantages of 70 Base Speed:
Compared to 85, gives a lot of room to improve other stats while still allowing a +Speed nature to beat Base 80s.
Disadvantages:
Ties many common pokemon like Metagross and Breloom, requiring a lot of Speed EVs if you want to outspeed them.
Advantages of 65 Base Speed:
Beats Swampert, Abomasnow, and Tyranitar. Minor investment needed t outspeed Skarmory.
Allows the most stats to be distributed elsewhere in the medium speed range.
Disadvantages:
Without Speed EVs, slower than Skarmory. Pretty much always slower than Breloom.