A little tough to say which mod was my favorite. There are really four basic categories: engine/driveline, suspension/chassis, appearance and rider comfort-related items. I've all but re-engineered my rider into a real road bike; everything but the main frame, rim halves, tail light, rear fender, folding bar clamp, seat latch & hinge, and fuel tank has been modded at least to some extent. It's been a lengthy, mostly enjoyable and, ultimately, satisfying pursuit. To me, being able to just fire it up and ride anywhere/any time, with stone-reliability gets the nod as top priority. Closing in on the 20K-mile mark, the only failure has been the Jincheng clutch cable, that was retained from the an earlier incarnation of the bike.
Horsepower usually hogs the limelight, which is understandable...just a little off the mark, imho. The engine is the heart of any bike and without enough usable power to keep up with traffic, a road machine is a non-starter. Beyond that level, however, is where I see the tradeoffs getting steeper while the returns get smaller. For that reason, I agree with you that a reliable 50mph+ is a good place to start. On that score, while I consider the 110 Nice motor the ultimate in bang-for-the-buck for a high-mileage road machine... without matching suspension, braking, and chassis improvements, 50mph+ cruising just wouldn't work in very many places in the great pothole state. Thus, I'd have to consider suspension, brake & chassis upgrades a virtual tie for the #1 spot. Forced to choose, I'd say they're a VERY close second.
Slight off-topic, but still very important, is rider accommodation. Seating, handlebar grip position, and even exhaust note can cause rapid rider fatigue, if not done right...what OEM vehicle engineers refer to as NVH - noise, vibration & harshness. Once the bike is fast enough to safely venture into traffic, it needs to be tractable at those higher speeds and finally, not beat the rider into submission...i.e. comfortable enough that you enjoyed the ride, not just withstood it. I've left this kinda vague & generalized; it's meant to be a sliding scale. Not everyone rides the same number/type of miles or in the same conditions. FWIW, here's a handy way to check your success...if every ride seems shorter than you'd like, mission accomplished.