debateable...imo you either want the longest stun possible and use slam, or you want to just give a short opening to peel and then you use the shortest stun possible so the immunity is short
But even considering some niche scenarios where 2s is too short and 9s is too long...having the 9s option changed to 5s duration is not a buff, no matter how you twist it
The optimal buff from a 8v8 perspective would be setting it to 4s i guess, but 5s is not far away from that.
4s = you can stun and snare without any possible counterplay (unlike numb, where the enemy melee can still snare you back, after numb+snare)
5s slam would be peeler heaven from a exclusive 8v8 perspective
and it would be still long enough to attempt to kill any target with both melee or caster assist. - If you don't kill someone within 5s, then it won't happen usually.