This was my response to your post on the US forum:
There IS an ability cap. It's just that nobody knows its exact amount.
What is more important than boosting is the decision of getting lots of vets on your fighters (at least 150). Are you ready to pay for that? If not, save yourself the boosts, too.
You can boost your fighters anytime, there is no difference to their ability if you do it early on or later.
At the moment, as your fighters are T2, you will not notice them becoming stronger. Unfortunately, they are still food for everybody else, and no boost will change that.