If you do not want to go to a bigger had, and you will not increase ground speed, there really isn't much reason for a bigger machine. 

With that said, if you are truly loading the 66 at the speeds you state, your heads have enough extra capacity to load an 88, but probably not a lot more.  I would expect the 88 to do 1/5th more, or perhaps a little better, in comparison to the 66.  

I would consider an 88 to be an 8 row, maybe 12 row and 25 or 30' machine, however I'm aware that your crops are quite a bit different there.  

We've picked lots and lots of acres of corn at 6.7 mph.  I've never had a lot of luck getting a cutterbar to perform as I want abpve 5 mph.  

I'm curious about going from 8 row to 6 row?   Was that a planter based decision?    We avoided going to a 12 row planter specifically because we thought it would screw up the combine too much because we really wanted 8 row heads.  Then later we found ourselves stuck with 8 row heads because of the planter.  lol.