RE: XX75 or XX78?
7078 alloy is considerably stronger than 7075 alloy and will resist denting and bending better than the 7075 alloy.
If I remember correctly XX75's are rated to 95,000 psi and XX78's are rated to 120,000 psi.
The xx78's are also heavier than the same shaft in xx75 though it is only like a tenth of a grain per inch.
The xx78 shaftsare also made to tighter tollerances than the XX75's.
AS for your delema, Easton does not list the 2016 in the 78's any more (which you probably already know and the reason for your post)so you are either going to have to switch to 75's or try another shaft in 78's. Personally I would opt for another shaft in 78. Such as a 2114. It is slightly lighterthan your 2016's but it is also slightly stiffer than the 2016 so you could easily make up the weight difference in a slightly heavier broadhead.