First up I am not sure about an answer to your question but I'm sure someone will be along shortly with an answer.
If you are looking for an option while you are waiting for Linux agents we have a product called LANGuardian which also does deep packet inspection and it can be integrated with Orion. The 30 day trial may be able to sort out some issues for you while you are waiting. You can install on a physical or virtual platforms and you don't need an OS as its based on Linux. Just connect it to your SPAN ports.
Online demo available at this link http://demo2.netfort.com/Orion/SummaryView.aspx?viewid=45&AccountID=guest
Let me know if you have any questions
NPAS vs. SPAS doesn't really make a difference if we're filtering traffic for the same nodes, we'll still have the same accuracy of response time data. Here's a quick video on how ART / NRT are calculated: http://www.solarwinds.com/deep-packet-inspection.aspx