What Vegas is alluding to is that in a street motor, making mods to increase the volume of intake flow could possibly kill the lower rpm port velocity. That would mean less cylinder filling, thus lower HP readings at lower rpms, though higher rpm power should be better. (And, lower port velocity mucks up the carb's ability to meter fuel properly. That's why "real" carbs have accelerator pumps!)
Add to that a "bigger" cam (longer duration and more overlap) and the problem of loss of low end power is exaccerbated.
Shades of those 10,000+ rpm crotch rocket 600 cc 4 bangers.
As for cams, though, IMHO, any custom grind for a Hinckley twin, which can get away with using stock valve springs, is not too big for the street.
One other note: Overall gearing of one's bike makes a big difference in what one can do for mods to his engine, and get away with. That is to say, if you're trying to pull an 18 t. sprocket, you're going to need more bottom end power. But, if you "gear down" to a 16 t. sprocket, bottom end power (lack of, actually) becomes less of a problem.
This whole area is a lot more complicated than I care to address here, but suffice to say, a nice mild custom cam, some port work and bigger intake valves, increased displacement, opened up exhaust system, and better ignition output won't kill one's street performance.
