There's no penalty from having more skill, it's just that your normal damage is higher compared to the bonus.

i.e...

100% = base damage

Your Tac Team adds enough skill to get +20%

you get a 20% boost in weapons damage.

compared to...

You already have enough skill to get +50%

Tac team adds +20%

you get a 13% increase in weapons damage.

So putting "+20%" would actually be less informative than the skill # boosts you see now, since it would be misleading. Additional, skills that actually add +damage (like Attack Pattern Omega) behave a bit different from skill buffs - Tac Team only impacts your weapons damage, +damage affects damage dealing powers as well.