Well a Jolly 252 EV on speed Blaziken has 284 Max Speed Stat
Since a maxed out speed boost would amplify this by 4 it would become 1136 Speed
And Tailwind doubles the speed stat so this would become 2272 Max Speed
Although you didn't ask this, if you were to scarf it on top, this would become 3408 Max Speed :)
However, reaching that level of speed is completely useless as you wold be outspeeding almost everything of not everything at x3 speed boost xP
