It looks like I can keep highway MPGs at a steady 50+ now.
I didn't notice any change in cabin noise with the VGs, but I did get some videos today of testing comparing the hatch spoiler, VGs, and the stock hatch.
The engine temps seemed just fine. It warmed up slightly faster than normal, but that's still pretty slow IMO. But it is only a 1.3L, so it doesn't make much heat to begin with.
The videos are uploading now, and I'll post them when they're done.