So to clarify for people, if this is Pierce for picks straight up, the deal can't be completed until after June 30th, since the Cavs don't have that kind of cap space available this season (no one does, due to minimum payroll requirements). Such a trade exception would be $15,333,334. The Cavs would obviously keep pierce, since this trade would occur after the deadline to release him and owe only $5 mil.
If the Celtics were to use the exception in a sign-and-trade, they could only pay the incoming player $15,333,334 or less, which is about $1 mil less than max. If someone like Josh Smith is adamant about getting a max contract, this will not be enough.
This deal makes some sense to me, depending how cranky Irving is about not losing. They have a gaping hole at the three, and if they don't end up with Otto porter at the draft, pierce would be a great fill.
Additionally, to sign-and-trade for the full value of their trade exception, the Celtics would need to be under the apron after the sign-and-trade. Currently they are projected to be over, so they'd need to clear some additional salary room. Logically, KG leaving is one way, and accordingly, I don't see this trade occurring unless KG is gone.