There's no penalty from having more skill, it's just that your normal damage is higher compared to the bonus.
100% = base damage
Your Tac Team adds enough skill to get +20%
you get a 20% boost in weapons damage.
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.
I think then it should be changed because it will be much clearer.
I mean can you tell me how much it boost weapon power when you already have 9/9 skill? Does it increase it at all then? Or is it 0.5% per point?
In every space skill I've tested, including those affecting weapons and science abilities, each point of rating (e.g. +1 Starship Operations Training, +1 Starship Energy Weapons, etc) does in fact add 0.5% to the bonus.
As the previous poster pointed out, adding 0.5% to the bonus almost never means adding 0.5% to magnitude of the effect, because there is already some bonus.
In general, effects seem to be computed as:
Final = Base * (1 + Rating/200)
Rating includes bonuses from skills, buffs (like Sci/Eng/Tac Team), deflectors, consoles, etc. Hence +1 Rating adds 0.5% of the base value.
For abilities affected by weapon power, it goes like:
Final = Base * (1 + Rating/200) * (Power/50)
For science abilities affected by aux power:
Final = Base * (1 + Rating/200) * (1 + Power/50) * 0.5