As far as I know, regardless if you use 6 or 8 engineers you will still hit the 40 knots speed cap by the time you can access nebby, unless you unnecessarily armor your nebby.

I have a nebby with 4 engineers, 4 repairers, 2 main and 2 reload gunners. It still goes 40 knots.

6 engys (if +12 rep stat also) 4 reps (enough for rep cap IIRC)  for longer overheat(my preference)

5 engys and 5 reps = waste of time, as you lose valuable OH time, yet still are at rep cap

tkz for ansing the question

:)) 

I'm hitting the speed cap easily with 4 Engineers, 3 Repairers, and 1 Seaman, plus using AA and and maintaining great overheat time.

The trick is a level 120 seaman with 300 vets.  Seaman ability is not capped, which allows me to hit the speed cap, the repair cap, and boost the overheat provided by those engineers.