Shooting the D800 in DX crop mode gives a slightly better IQ than the D7000 withthe same lens.
From curiosity I decided to find some of the lenses mentioned making it failry easy to compare size, weight, and price. Basically if one of the first three options do not work, then the expense quickly climbs to $3,000+ along with at least doubling of the weight from the 80-400mm.
Lens type Size, weight, price
Sigma 150-500mm f/5-6.3 DG OS HSM APO, 3.7 x 9.9 in, 67 oz, $1069.00
AF-S NIKKOR 300mm f/4D IF-ED, 3.5 x 8.8 in, 50.8 oz, $1494.95
Sigma 50-500mm f/4-6.3 EX DG HSM APO, 3.7 x 8.6 in, 64.9 oz, $1659.00
Sigma 120-300mm f/2.8 DG OS HSM APO, 5 x 11.4, 104 oz, , $3,199.00
AF-S NIKKOR 300mm f/2.8G ED VR II, 4.9 x 10.5 in, 102.3 oz, $5889.95
AF-S NIKKOR 200-400mm f/4G ED VR II, 4.9 x 14.4 in., 118 oz, $6999.95
Sigma 300-800mm f/5.6 EX DG HSM, 6.1 x 21.4 in, 207 oz, $7,999.00
AF-S NIKKOR 400mm f/2.8G ED VR, 6.3 x 14.5 in, 163 oz, $9549.95
AF-S NIKKOR 500mm f/4G ED VR, 5.5 x 15.4 in, 136.9 oz, $8,579.95
AF-S NIKKOR 600mm f/4G ED VR, 6.5 x 17.5 in, 178.5 oz, $20,299.95
For many of these a teleconvertor would be helpful. AF-S Teleconverter TC-17E II, 2.6 x 1.2 in, 8.8 oz, $549.95