I've only read from page 3 (wheyyy) but can't really understand the issue. The bike runs a little RICH allegedly from standard (not lean, that would create possible damage and not produce good power which would make no sense). It was made in the era before emission policing grabbed bikes by the short and curlys. You can smell they are quite rich, mine smells more fuelly from the exhaust than my dads SV yet gets better MPG. To cure this little bit of "richness", a lot of people (myself included) fit a K&N air filter for about £35 and an exhaust can of your choice. The K&N is pretty cost effective if you plan on keeping the bike a while, it's washable and has unlimited warranty. It's a premium product and fits much better than the standard filter I replaced which was black, gummed up and made a weird mayo sludge in my airbox (dealer claimed bike was serviced when I bought it....) Never had the problem since with the K&N.
As for the exhaust, I don't see how that makes much difference unless you have the baffle out and are over 8k or something. I guess the simple theory is with the K&N you are getting more clean air into the engine, and with an unbaffled straight through end can the exhaust gasses are being expelled faster than the stock can allowing for a more efficient combustion. I don't know how it all works other than basic knowledge, but that makes sense to me.
As for yours, 81bhp is definitely fine for a Fazer at the rear wheel. Some will be more, some less. You can spend hundreds on jet kits and dyno time but a lot on here have tried and not had good results. Standard is the best all round. There is a flat spot yes, for me its about 4500-5200 it just doesn't quite have the go but then after that it's off and pulling hard. Not sure what sort of advice "go and drill holes in your air filter" is, but no just no. If you mean air box, that is also not a great idea and pretty old school bodgery that is all the rage in your local macdonals carpark chav corsa drivers car, along with about £400 worth of halfords anodized tat.
Fit K&N, fit exhaust if funds allow, decide whether baffle in or out (out probs better for high rpm but can make commuting low rpm a bit fluffier), balance carbs again and go and give it a good blast to try and clear it out a bit. Keep it serviced well and all should be great. I'll go and read page 1 and 2 now.
