Your theory seems reasonable. I've heard "vague steering" comments about Versas, and that may factor into the tracking issues too; i.e., a balanced/heavier load may help mitigate that. (For the record, I haven't experienced it.)
When I read your question I was thinking of crosswinds and didn't realize you were referring to big rig turbulence and the like. I haven't noticed any change in response there, and it's no worse than other small/lightweight vehicles I've driven (and definitely better than an empty cargo van!)
It seems odd you'd be having this issue in your heavier sedan (albeit only a little: 2516 pounds vs. 2477). Granted yours is a bit longer (176 vs. 169 inches) but I have that stubby extra side area (above your trunk) to catch the wind. I'd think if either car was going to be buffeted around it'd be my lighter, blockier hatchback.
I'm going to test a partial kammback eventually (see a photo of the preliminary test in my garage -- and yes, the angle was too steep). It'll be interesting to see if that additional high side area makes buffeting more noticeable.
How much has the added weight affected your fuel efficiency, if at all?
A thought: Instead of 150 pounds of nonfunctional weight, how about including a deep cycle AGM battery or two and going for an alternator delete?
Cheers,
Rick