You can certainly count on improvements to the preview render speed and usability for the final release. Although ideas like this may not be directly implemented due to various feasibility problems, I think you'll find the final preview system to be pretty effective given the trade-offs involved.
Comparing to preview systems of other applications, and given the assumption of further optimization, I think TG2 does fairly well as it is. We could potentially use a simpler and faster system (keep in mind that you can already turn off realistic lighting and atmospherics to speed up the preview), but then you'd have to do more tweaking and test renders in many cases because the preview wasn't as accurate. So it can be an issue either way, really. I agree that providing the option of a faster but less accurate preview would be nice, but given the tight development focus and timeline way may need to focus on the single existing preview and just make it better/faster.
- Oshyan