The fella who used to Dyno my bike years ago when he used to clean my carbs for me always said that the original downpipes were the best, without question, but obviously trying to find a decent 2nd hand set could prove a bit much in this day and age, and new will probably cost the earth.
As for end cans, shorter will simply be noisier. And moneywise there is quite a big range to go through.
I have a short SP Engineering can which is not only noisy as feck when it is legal with the baffle in, but with the baffle removed it just takes the piss.
I now stick with my longer Quill exhaust which has a nice low down rumble when riding normally around streets, but does get a lot noisier in a nice way when you open up the throttle. They're not cheap though.
Some things to bear in mind though:
1. It may sound louder, and feel faster with an aftermarket exhaust, but it's all in your head, unless you really go to town on tuning up the whole engine. Not that it matters, the grin factor will still be there. I was never a fan of the original cans underwhelming sound either, and changed it almost immediately.
2. I personally would recommend using a K&N air filter as well if you change the exhaust system, giving the bike more air to breathe, it can be quite noticeable on throttle sharpness.
3. Regardless of what manufacturers say, a carb balance is recommended after it's all changed over, especially if you choose the K&N air filter too.
4. I've no doubt you've read on here about peoples woes undoing exhaust header bolts before. I had the same pain myself last year, the 6mm studs are either stuck solid to the nuts, or the stud will just fall apart in your hand. Give yourself half a chance of success by keep on using a spray penetrating oil way before you even think about removing them....like now for instance....to help loosen any rust that will cause issues.
5. As for exhaust systems, plug or Bluetooth a phone/laptop/tablet into a speaker system, and look on YouTube, as there are loads of Fazers on there to give you an idea of sounds, but you need a good sound system to hear them properly.