I sprocket change, either front or rear will be the easiest and probably the simplest.
You can do one (or a combo) of things to your sprockets:
Decreasing the tooth count on the front sprocket will
increase your "low end" torque at the cost of "top end speed"
Increasing the tooth count on the rear sprocket will have the same effect, that is
increasing your low end torque.
This works conversely as well.
It's important to note that changing the front sprocket tooth count will have a much more dramatic effect. "Dropping" one tooth from the front will make a huge impact. The rear sprocket will require more teeth to "drop" or "add" to see the same effect.
Of course you could go with a 12 tooth front and 58 rear (stock being 14/48?) and have monstrous torque, but it would be a horrible ride. Others here have experimented with sprockets on the FZ and can comment more smartly than I.
This is a tool I like to use, it has several (many) pre-loaded bikes that you can choose from to input the stock gearing. From there you can play to your hearts content:
Final Drive Calculator
YES, changing the sprockets will have a corresponding effect on your RPMs in each gear. How dramatic an effect depends on what tooth counts you use on your sprockets.
While some exhaust systems are out there purely for sound or "show", most of the ones you see for the FZ are going to have some kind of impact on your horsepower and torque numbers. To get the most bang for your buck with aftermarket exhausts you really need to run your bike on a dyno and have someone with the capability and knowledge to tune your fuel mapping. This is also going to require some piece of aftermarket electronics like a power commander.
