I like the idea of brimstone's stim beacon because it fits his character and theme, but it is kind of cheesy and lackluster in actual games, because attack speed alone is situational. That being said, I like brimstone's teambased aggression that comes with insta 3 (4 if you count ult) smokes and buffing your teammates to rush and high tempo play.

If brimstone's stim beacon gave a movespeed buff similar but weaker hyper speed buff without the jump height, brimstone on attack would be a force to be reckoned with, due to how much map control and speed an execution can create. On defense, brimstone imo is better as a retaker instead of a proper defender like the sentinels or viper, playing around that type of playstyle definitely helps. It also gives a distinctive playstyle to each controller, whereas right now, omen in practice is better than brimstone thanks to his powerful flash and rechargeable, longer range smokes.

Each controller should fit a certain specialty makes for a more varied and interesting game.

1.Brimstone = attack/execution based. Rush with guns blazing, favors head on approach.

2.Omen = jack of all trades. Subtlety and tricky plays are part of his kit.

3. Viper = defense/post-plant based. Force enemy to push in less than ideal circumstances and potential get caught with their pants down or respect her utility.

I don't know what specific percentage would be considered OP or game breaking, but starting it from 10% movespeed buff could be a start, and tweak the buff duration to be shorter. Covering a lot of ground during an execute or in general either in preparation or retake or rotations feel like it has far more use than just attack speed. Actually, even trading attack speed for movespeed seems nice.

This might have some technical downsides on the receiving end, like peeker's advantage being worse than usual, but if I am totally honest, if they shoot you before you can react, that movespeed doesn't really help (ehem OP peeking), and you simply need to adjust for wide swinging speed. It might even be good practice against jett ults, since she runs at knife speed anyways.

TL;DR Add movespeed to stim beacons. I'm just saying having 2 vestigial stim beacons rn in place of something more useful seems like a waste of an item slot, but I like the idea of stim beacons, so it should give a consistently useful buff. Attack speed is situational and it only gets worse as the ranks go up, where people know how to headshot consistently.