If you compare the MC16 to it's RS relative, the basic design is the same, but the porting on the NSR is very reserved. I suspect that this is the case with the NS250.
I would imagine that the only way to extract more power is to pull the barrels off and get in there with the grinder. Then you'll need pipes, and probably carbs......and then an ignition.....and so it goes on.
I'm sure that when Honda built the NS250 they wanted it to be reliable and reasonably easy to ride. When they built the RS (of that era) it had to win races. There's a huge difference.
I don't have the 1984 port specs for the RS, but I do have the 1985 specs which may be similar, so if you get the chance to whip off your barrels we can have a look to see the differences.