I suggest skipping the slip on, and just going where you want to ultimately go. The best full exhaust you can find (for a bike that hasn't been made in almost 12 years), and PC. You know you will eventually find your way there, might as well save the few $$ you would have spent on the slip on (You can pick up a PC for the price of the slip on).
I made the mistake of not going all out from the start, but to be fair I got my slip on back in 2009, used full exhausts were still really really expensive back then. I picked up a used full muzzy for almost nothing in 2015 (there isn't a big market for them, so it is a buyer's dream). Come to think of it, I never sold my Yosh slip on (it's somewhere in the house).
Because of the age of these bikes, there is very little market for parts, I also picked up a set of Corbin Beetle bags for a steal $500 shipped (originally new these were almost $1,700).
2005 ZX12R - Sold
2009 Honda VTX 1300R