Not as much traffic here as I expected!  Lacking advice, I installed both Vue 6 Inf and Vue 11 Complete on the new machine and did some tests.  The results are not quite as simple as I first thought because I hadn't taken into account the optimisations to the renderer in Vue 11 vs Vue 6.

I thought that the information might be useful to anyone else who is thinking of upgrading either their machine or their copy of Vue.

Test 1: (4 spheres of different materials on a checkerboard):

Vue 6 Inf: 15 sec            Vue 11 Comp: 23 sec

Test 2: "Glass Horse" scene from Vue 6, rendered at 1024x768

Vue 6 Inf: 21 sec            Vue 11 Comp: 1 min 16 sec

Test 3: "City" GI Atmosphere with a 49cm Sphere with "ruby" sub surface scattering material, rendered on Final at 1920x1080

Vue 6 Inf: 22 sec            Vue 11 Comp: 6 sec

Test 4: "City" GI Atmosphere with a 2m Sphere with "ruby" sub surface scattering material, rendered on Final at 1920x1080

Vue 6 Inf: 1 min 11 sec            Vue 11 Comp: 59 sec

Overall, until I can afford an upgrade to Vue 2015 Inf, it looks as though I am better off using Vue 11 Complete because of the better optimised renderer.

As a final comparison, I rendered Test 3 and 4 on my old machine to justify buying the new one to myself:

Test 3 (Vue 11 Complete):

i7 5960X (3.0 GHz): 6 sec                i7 2600 (3.4GHz): 12 sec

Test 4 (Vue 11 Complete):

i7 5960X (3.0 GHz): 59 sec                i7 2600 (3.4GHz): 2 min 27 sec

i7 5960X, 32GB RAM, GTX 1080Ti, GTX 980 Ti, Windows 10 Professional.  Running Daz Studio 4.11, Poser 11, Vue Inf 7, Photoshop CS4