Alright so an update here. Did a 51 mile ride this past weekend (excluding ferry rides). Burlington, VT area to Plattsburgh, NY. Mostly paved.
1) Setting the display to 9 PAS levels vs. the 5 stock seems to tame the motor a bit at level 1 and makes it pretty good for road use. I was using level 1 the entire trip. Maintained about around 16mph, except on some hills, but that was fine... I just shifted in those cases. I could have increased the PAS and the motor would have no issues maintaining speed (or speeding up) but was going for economy.
2) The throttle is pointless in PAS 1. It does next to nothing.
3) Managed the 51 miles with, supposedly, 37% left. That said, the capacity percent definitely drops off slower close to 100% than it does as you get lower. I don't know how that changes below 37% though. I need to toss the battery on my other bike to see what the actual voltage is reading. Bottom line is 50 miles was no issue. There was a 10-15mph steady headwind on the way out, and a bit of a tailwind on the way back, but it wasn't as strong. The route was flat'ish (google has 827 feet of elevation gain, round trip). Temperature was in the upper 50s. 21ah battery.
All in all, the bike worked really well for this kind of a trip. As expected, for road use I pretty much only needed PAS 1. Using it at that level, for the most part, isn't overpowering. There is plenty of additional assist available for hilly or faster stuff, if you want it. My guess, after this and how I understand the M510 works is the M510 may be a better choice for primarily road riding, but the M600 is definitely a reasonable motor too, especially if you were to want more power at times.