Ok, my turn - lol. Maybe it's best I don't get started on this one, but I'm bored right now waiting for my new front forks to arrive.
First, while there may be rare instances where this is NOT true (i.e a poorly tuned bike to start with), but for the most part you can have good gas mileage OR you can have performance, you can't have both simultaneously. The reason for this fact is that to get better performance you are squeezing more air/fuel mixture into the combustion chamber. The better exhaust pipes help clear it out so more can get in, and the bigger carbs allow more in. There are other components (camshafts, pistons, etc) that have a say in this process, but I only see pipes and carbs discussed in the previous posts.
Now on to the the air/fuel mixture. In a diesel engine this ratio is held constant. This explains why diesels get the same fuel mileage when they are towing and not towing. This is not true on gasoline engines. The air fuel mixture varies on loading, acceleration, and many other factors. By changing the pipes you are altering this ratio since more of the bad, spent combustables will exit the combustion chamber. This means that you will have to pump more of the good stuff in. As Slowpoke stated, you will be running lean. Lean is NOT good because lean engines run hot. This heat can harm your engine and make you wish you had never messed with it.
Carb re-jet or new carb? If you are looking for a slight performance upgrade, rejet your carb with the pipe you want. This will give you the looks and sound (may or may not like) without costing a ton of money. OR go whole hog and purchase the new carbs. Keep in mind, however, that you will still need to jet (or re-jet) them to get the correct air/fuel mixture. This will cost considerably more and I doubt that you will get much more performance since you are not altering any of the other engine vitals.
There is an old addage in racing - speed costs - how fast do you want to go? I'm relating this because new pipes and carbs really need better cams, more compression, blah, blah, blah. All of this costs lots of $$. Especially if you have someone else do the work and it sounds as if that's where you are. If you have lots of cash to play with go for it.
If you decide to do all the work yourself, the pipes are a bolt on - no problemo. To jet your cars properly, however, that is a bit of a trick, especially since we are dealing with 4 synchronized little buddies. First, syncronize them. Then go to a nice straight spot on the track, road, wherever and go flat out - open throttle. Then shut it down and read the plugs. They should be a light brown. If not then ad bigger jets if the plugs are white or smaller if black (I doubt if you will be rich to start with, but you may go too far). It's an iterative process and I'm not really doing the science justice here. Get a book, it will explain it in more detail with pictures too!
If I were you I'd either leave the bike stock or just add the pipes and re-jet. But I'm not, so whatever you do, good luck.
Steve
