As to the RTA, I'm clueless about the technology but I'd like to get a measure of how much quieter before vs after dynamat

Then RTA is overkill. RTA's job is to measure the details of the frequency spectrum. To get a proper comparison pass, you'd need precisely the same settings in SpectraLAB and on the PC's sound card inputs, and precisely the same microphone placement for each pass. Then you'd need to make certain you measured at precisely the same speed/RPMs on exactly the same section of road.

I'd just get an SPL meter and use that, to reduce the number of variables you need to account for between the two passes.