I went with the 7840HS. If I could have been sure the FW 16 would support the undervolting functionality that the 7940HS has, I might have considered it, but since that wasn’t a sure thing, I decided to save the money.
Has it been confirmed one way or another if the FW 16 will support the voltage curve optimization (or whatever it’s called) “out of the box,” so to speak? If so, that might be a factor to consider. I don’t know that I’d cancel and re-do my pre-order over it, but for anyone who hasn’t ordered yet, it may be a consideration.
Technically, despite the big reported gains in efficiency from undervolting, there’s really no guarantee it will even be worthwhile in every case on every chip. So the only thing you can really count on is the bit higher base and boost clocks.