For the price of single M6000 (4-5k$) You can build entire rig (CPU+motherboar+RAM+PSU+CASE+Storage) for rendering in Octane with not one or two, but three TitanXs =) this build will have same amount of effective VRAM, but 3 times more render power without any negative side effects that You would notice in Octane.
if You're not planing any other specific workloads for that card like computation that requires dual precission performance, 36bit output, etc. You're much better to go with TitanXs for Octane Render. Those will give You 120-130 on air, & as as an option around 150 under water in terms of OctaneBench, with much lower temperatures, close to none of noise.
p.s. for the price You would pay for pair of those M6000 You can build custom PC with 7 TitanX that instead of 250 in OctaneBench would give You ~1000 (4 Times more render power =) so..Yeah..Don't Even Think About M6000 for OR!