I'd say a module which reduces your own radar range to 500m, reduces the range you're detected by to 250m and reduces your speed by 50% would be interesting.
Locking any target or activating any other module would cancel the effects immediatly ( module would finish the cycle, but the effect granted by it would not last until the end of the cycle. )
You could sneak up on people but the moment you'd start locking they would detect you and be able to fight back. In addition to this because of your own reduced radar range there could be enemies just outside that range to surprise you.
Say you use the module, get within 300m of someone, you can see them but they can't see you. 300m behind that person is an ally of theirs ( so 600m from you, meaning you can't detect them. ) As far as you can tell there's only 1 enemy in range. As far as both of them can tell there's nobody in range. If you decide to engage that enemy radar ranges go back to normal and suddenly you see 2 enemies and they see 1.
Or instead of making penalties to speed why dont we make it simple? Like a lightweight frame you could have a frame that reconfigures your bot to be "stealthy" like radar stealth is in the real world with radar absorbing surfaces and so on. This -could- be in place of a regular lightweight frame, I dont quite know how the balance is. If you spot the stealth bot with your own eyes your should be able to target it but it shouldnt appear on your radar/landmarks until its too close for comfort.
On a different note, I cant say i like the concept of cloaking devices that make you completly invisble, I kinda like the much more plausible low tech approach and I think it would work really well in a game like this as you would need to think about the terrain you are sneaking in, while still being viable for surprise attacks and ambush tactics (a moving bot is much more visible than a stationary one)
