but the amount of combat (distraction, scouting etc.) and RP possibility (mimicry) of such a pet can be very high
No it's not. Not by a long shot.
distraction
If your pet is in the middle of combat, it's going to get killed. It only has 1 HP and 12 AC.
scouting
Any familiar can scout! That's the whole point of having a familiar!
In terms of scouting, I'd rate ravens medium-tier. They can fly sure, but they're out of place whenever you're indoors. Rats and spiders are much better scouts because they can plausibly be just about anywhere, so even if they're spotted, the enemy has no reason to care.
RP possibility (mimicry)
Why the hell is this a reason NOT to let him have it?! Do you not want your players to do cool RP stuff? Besides, the DC is fixed at 10 and never increases.
A better question for you is which Familiar ISN'T OP? An Owl has +3 on Stealth checks, +3 and Advantage on Perception, 120 feet of darkvision, and has Flyby so it can use the Help Action in combat while staying out of reach of being hurt. Is that OP too?