In fact, there's one species that can get more than a 50% speed boost from the item slot: Ditto, if it's holding Quick Powder and hasn't transformed yet. By Baton Passing +6 Speed to Ditto and immediately hitting it with Speed Swap (on something that's already swapped its speed with max-speed Deoxys), Skill Swap (giving it a weather or terrain speed boosting ability), and making sure that both the appropriate weather and Tailwind are in effect on its side, a rudimentary calculation would tell you that Ditto has speed of 16,128 at this point.
However...it doesn't actually get to make use of all that speed, even for transforming. The last few steps of speed computation, in order, are:
- If your speed is greater than 10,000, reduce it to 10,000.
- If Trick Room is in effect, replace your speed with (10,000 minus speed).
- If your speed is greater than 8191, replace your speed with (speed minus 8192).
So in Ditto's case, the first of those steps will cut out the excess and make it fall to 10,000, and at the last step it's still too high and wraps around to 1808, which is all it gets credit for. As an example, see this battle, where we construct just such a Ditto, but since the naive maximum speed really only counts as 1808, it manages to get outsped at the end by a Swoobat that "only" has 2512 speed.
The maximum effective speed you can possibly have is 8191, as anything slower gets outsped and anything faster wraps around to 0 or other low numbers in the final step. However, the only way to actualize this highest of perches is to get a speed that would otherwise be exactly 1809 (such as a starting speed of 402 at +4 with a Choice Scarf) and then put Trick Room up. Without Trick Room, it's impossible to land exactly on 8191 no matter what starting speed or combination of modifiers you try to use--the closest you can get is 8190, from 455 at +4 with Choice Scarf, Tailwind, and a weather-speed-boost ability.