Joruk: On the flats I've never been able to hit 30mph, but I also haven't tried too many times. It's way it's easy for me to get to 27 mph on the flats (and I'm a big guy 6'2" 260--a fairly fit fat guy!). But I would find a motor cutting off at 20 mph to be annoying, because it's really easy and comfortable to ride just above 20 mph on the bike. I don't know that I could do 27 mph in work clothes on my way to work, and not get there needing a shower. I'm pedaling hard to get to and maintain that speed. Also, while I'm comfortable going downhill at 35 mph on the SL, pedaling hard on the flats to 27 mph does (for me) get close to my comfort level in terms of stability and maneuverability. Not sure I'd want to have to maneuver in traffic to avoid a surprise pedaling/exerting that hard at that speed. I've just done it on farm road in the country side (because I can and it's fun). Hope that answers your question, at least in part.
But I'll also add that since you're a more accomplished cyclist able to ride long distances at 20 mph without fatigue on a regular bike, then I'd think 25 would be easy for you to do on the Vado SL. I know I always feel like I could do 18-21 mph "all day" on my Vado SL on the rolling flats (not literally all day--but clearly for extended periods of time without excessive fatigue).