The NSR150SP has a reasonably small exhaust port, although it's timing figures are quite clsoe to the NSR250. It struggles to flow enough gas over 10000rpm to make decent power so to be honest it needs a bit of modification all through the motor to make respectable power.

If you pull out the reed valve you'll see that the shap of the inlet is a tad on the small side. have a look at an RS125 or CR125 motor and you'll immediately see the difference. The latter two are high hp, high revving motors. The 150 is not. Take the intake of either of the race machines as a guide to modifying the NSR150SP. In fact the older NF4 RS125 is a very good place to collect data with a view to modifying either the NSR150SP or the NSR250.

The 150SP exhaust port doesn't need any more duration, but it does need more area. Consider thinning down the centre bridge and lifting the outside edges. The transfers are on the same boat. They don't really need more duration, but you can increase area. The nikasil on the edges of the ports is also rather nasty on the 150SP, so clean this up. Remember to leave a nice chamfer on the ports to give the rings an easy life.

The actual exit of the exhaust port into the pipe is also too small for big revs. A fraction more area here works well at the top.

The head is designed for low rpm torque, but still gives good top power. Consider less squish area (NOT to be confused with clearance) for more top end.

I can't give you solid numbers to work to, but by having a good look at any race engine will give you a good idea of where you should be heading when you want to up the hp of a two stroke motor.