TJR still modifies the OEM Nice head for 28.5/25mm valves with reduced-diameter (4mm, as I recall) valve stems. And you're right, the ports are huge, especially the intake. That said, Boyle's law still applies; increasing port volume decreases flow velocity. On the street, with anything smaller than ~160cc, that makes for an inefficient head at low-to-midrange rpm, where oversized ports are a liability.
15 years ago, things were very different. A 60mph bike was a milestone achievement, so raising the bar to 70, 75, 80+ became a goal unto itself. Fast forward a decade, or so, horsepower has never been cheaper. There's also some field service history out there and the tradeoffs have shown themselves. This discussion has been refined & focused, over a number of years...on real-world, 5-digit, road mileage. With 60-65mph on tap, almost plug & play, from a bone-stocker, it's now about the balance of miles and miles per hour. It's a balancing act, always has been.
The main issue, imo, is that all of the roller heads that fit these engines are aftermarket race parts. They weren't engineered to deliver OEM service life. I agree with Jarred, there is no real airflow volume advantage of the Nice head and the valves are small. That said, it's the small ports, valves and combustion chamber are where it is impressive...for the efficiency. And, since it is an OEM Honda part, it was engineered to for long service life as well. Efficient operation matters on the road...a fat & flat torque curve + crisp throttle response + high mpg numbers = usable power. It's a question of how much airflow (and with it hp) one has to trade to get the known longevity.
In a (coco)nut shell...as mph & hp increase, so too do costs - initial & longterm cost-per mile. The only thing that decreases is mileage between rebuilds. As a former mentor once put it, engine service life can be thought of as the liquid contents of a 55 gallon drum, tuning for more hp like installing a larger tap...that will empty it faster. Where do you want, or need, to set the balance point? IMHO, 50-60mph is real world cruising speed on these little bikes, when they're well set-up. Anything much beyond that is only good for short bursts and 70mph+ isn't as enjoyable as it might sound. Dealing with substantial grades ~10%, or better, over some distance, is where added grunt goes from a want to a need. FWIW, Jarred's setup, on this bike, is the kind of conservative approach I'd use for a (somewhat) limited use road bike. It should have no problem pulling 55mph up a 10% grade.
