I look at economics, a D600 or D700 is $2000.00 or so, the D800 is $3000.00, if you use it for 5-7 years then the extra $1000.00 is only 200.00 a year or so difference.
I used my D200 and housing for 5 years, and now upgrading to a new housing. So when you consider, 2 strobes at 750.00 each, and housing at 3500.00 approx, I look at getting 5 years out of an housing. I have all the ports and gears for the lenses.
I don't plan on upgrading my D800 and housing for at least 5-7 years or more.
I plan to buy another D800 or D800e, in the future as a backup.
I have 2 D200, which I still own with 2 housings, and 3 Nikonos strobes, which I'm keeping, because what are they worth to sell
I still have my film cameras, Hasselblads, Leica, Nikon F100's etc.
If your spending 2k, what's another 1k, you already into an expensive hobby, reef tank.