I have my GTX580 as the card for the OS (monitor connected) and a GTX680 (which has more GPU's, 4GB VRAM and 144 RGB slots) as the card for Octane. So I do not have a performance problem. But I guess it depends on what you use the system for. If you have a reasonable videocard, OpenGL will work just fine. If you play games however, that might not be enough (but you may be able to use a monitor switch)

In my setup I have the option of rendering faster with additional GPU's by activating the second card as well. But it has 1.5GB VRAM and then has to share it with the OS - this means it can only be done for small scenes.

The 8x and 16x thoughput does not matter too much for OpenGL (I think), it is only benefical for moving large chunks of memory to and from the videocard (games)

Once you get used to it how it works, it is pretty easy. If you modify a conversion for a particular figure or prop, you can save the Octane material in the Poser material. Save that in the scene, in the library or as a material/material collection and you can reuse it - saving you a lot of time. You can also save figure materials as Octane materials and reuse them for all your figures. Applying it has the option of retaining the texture maps which means you can reuse a saved material to any figure of the same type (gen4 for example). It will get all the SSS and other goodies you have setup with the new textures

There is one thing which I noticed however with skin textures. Your settings for those textures depend on the texture itself. Some translate very well, some need adjustments and some are not good at all. (but they did not look good in Poser either). So even though you can reuse them, you still may have to make adjustments depending on the lighting you use