Neither passing IBT not not passing it means stability or instabilty, you can easily pass IBT and fail Prime95.
In no way any normal use of your PC wil produce so much heat as IBT does, and rarly reach Prime95 temperatures.
Prime95 is perfectly sutable to test the system with high probability of stability (stability is never guaranteed by any software).
Rendering won't get to temperatures high as Prime95.
As of Cinebench results I don't believe half of it, at best 2600K will do 20 percent better than 2500K. Don't believe anything you haven't seen in your own eyes or at least on what looks like reliable video.
What score did you get from the FX-8150?
Ypu worried about Xeon board price, what about the CPUs?