I think the EMP is in a good spot in therms of how strong it usually
is (its average effectiveness if you want to call it like that).
It is just that it effectiveness goes from one extreme to the other and it doesn't have that "Oh sh*t, I got hit!" feeling the other items have.
I think the best way to solve this is to compress its effect, making it stronger but shorter.
Something like the wheels locking up because the EMP disabled the magnetic suspension and the wheel hubs touch the cars body.
Just imagine a lot of sparks spraying out of them because of the high friction.
It could handle like suddenly using the handbrake (the car is still steerable at high speeds but slides around when it slowed down quite a bit).
This would require a reaction form the player to angle his car to not slide off the track and reduce the time the EMPs effect needs to be active, because it slows down faster, so it would achieve the same thing in less time. Additionally it could be more satisfying to the players getting hit because they can reduce the disadvantage they will suffer from getting hit by the EMP by angling their car properly before they get hit.