factory pipe > coffman > westcoast > stock.
There is also a way to modify your stock pipe for more power if you're a welder.
With your stock 89 650sx, you have the 28mm keihin carb. Do the carb first. You need an intake manifold, carb, and flame arrestor. You will need to tune the carb when you put it on, so start reading about that now. Expect the carb/intake/FA setup to run you $150-250. The pipe might be in the same range.
After that, when you replace the pipe, you'll want to tweak your carb again. Lean seizures are no fun.
Then, you'll be hooked on modding, and you will want a prop, ride plate, high compresison head, advanced timing, dual cooling, light flywheel, light pole, finger throttles, paint job, aftermarket CDI, and eventually, a different hull. Welcome to the club.