If you go for Poser 8 you will loose the ability to render with the P4 render engine. That is something you might use. PoserPro still has it, but the successor probably not.
Document viewport size in Poser 8 depends on the UI. If you have a docked preview window it takes it size from its surroundings. If you set it to floating it keeps the same size until you change it - it is now part of the UI settings. It does not take its size from the scene file anymore.

Gamma correction in PoserPro and Tone mapping in Poser 8 are both optional features. If you do not turn them on, older scenes will render the same as in previous versions.

I still use scenes from 2004 and update them with the new features to make them look better, so loading or importing is not a problem either.