First post from long time lurker.
I have a 98 550 Zephyr. As you may know this bike has the same engine as GPZ550 and a cam change woul see some more top end hp (from 50 to 70 I believe). In addition some pod filters and rejetting helps.
Kerker made some pipes for the GPZ that fit with minor modifications.
My recomendation is also to fit some better progressive (icon?) springs and decent oil in the forks as this transforms the bike. ZRX1200 and XJ1300 straight fit rear shocks is also a good mod.
I really like this bike and have trouble letting go even it has not been really ridden for a year. Check out 'zephyrzone' and 'all things 550', really good forums.
Cheers from a norwegian living in Melbourne, Australia riding a 04 919 and a 2012 Ninja 1000