The purpose of the Affinity mask parameter is to attempt to prevent FSX from using the core0 of your CPU, where most of the Windows tasks are also running.
In FSX, using an affinity mask value like 14 for a quad-core CPU often gave a small increase in smoothness.
In P3D however, especially with the later versions, the effects of this parameter might vary a lot and sometimes might even be counter-productive.
From my side, with P3Dv3, using an affinity mask would often increase the issue of the blurries (the terrain textures not loading fast enough when flying fast).
In P3Dv4 I ended up not using any affinity mask anymore.
In the end, if you are using P3D already, I don't think it's worth the effort to keep FSX on your disk... Excepted for ONE reason: to keep the files for some of the FSX native planes to transfer them back to P3D where there might not exist. I'm thinking mostly about the Goose, the Beaver, and there was another nice one I forgot
These aircraft will look awesome in P3D, thanks to the improved graphic engine (new lights, new shadows etc...) and they work perfectly too
That's why, back in my P3Dv3 and v4 days, FSX Steam edition was still installed somewhere on my computer: only to get some of the default planes that were missing from P3D