I have a little predicament here along the same lines...
I will get round to doing a big system upgrade before the end of 2015, probably to X99, because pCARS will probably benefit more from extra cores than the Skylake improved IPC.
But the dilemma I have is regards this Q9300 system. My upgrade means my better half could take this system off me, to replace my old Opteron 165 / Asrock Dual SATA 2 system, in which case activating the Windows 10 upgrade makes sense.
But part of me thinks that because she will then have her current AMD 7850 and this R7 265, which can be Crossfired, it would be another factor to sway towards her getting a new cpu/mobo that would not bottleneck the Crossfire. In which case activating the Windows 10 upgrade on this Q9300 would be a waste.