Yes, it can be a bit hit-and-miss getting the jetting right. When this used to be a really popular thing to do, I think people had a better idea of how much to go up on the main jets for any given bike - with more people having done it, there was always someone around, whether it be an owner or a dealer, who'd be able to tell you what size to go with. Also, more dealers kept a good stock of main jets.
If you had a half-decent performance exhaust and
did get the jetting right, you would definitely have an improvement in performance, at the expense of fuel consumption - nothing's free!
To fine tune, you may also need to change the needle settings. I would think Dynojet would have a kit or be able to advise on this. If not, gotta be someone out there who could help. Once you
do have the air box out, it's much easier to pull the carbs off, if you need to change things more than once before getting it right.
Just looked at the Dynojet site - they do a Stage 3 kit which is for bikes with K&N filters and performance exhaust
for some bikes. Unclear as to whether they do one for the Fazer, but you can contact them via e mail:
technical@dynojet.co.uk or phone:
01995 600500