[Andreas]

I don't really see why concentrating while grappling should be that difficult. In my experience it is no harder to do so while grappling than while engaging in high intensity sparring while standing (though I do admittedly have more experience with grappling).

If you impose a penalty related to the opponents strength, it should definitely take your strength into account as well. A child clinging to me won't impair my concentration nearly as much as an adult. Also even for somone much stronger than you there should be a cap for that penalty. There is a limit to how much you can distract someone with grappling without wounding your opponent (which gives it''s own penalty).