The "5 rig limit" is per performance, and, more accurately, it should be said that each performance has 5 presets available to it. You can have hundreds of performance set up, giving you hundreds of rig options. You really should read up in the manual instead of searching and copy/pasting results from the reference PDF, because it really is a powerful option.
You also do not need to use it where 1 performance = 1 song. As a matter of fact, I find this the least intuitive, because you end up with lots of duplication. I have an external MIDI controller (RJM MMGT 22) that I use to control the Kemper (as well as several other devices). I use the performance slots to group together rigs - clean rigs on one performance, edge of breakup on one performance, "artist" rigs on another, etc. However, you do not need to stick within guidelines for it - it's basically a way of loading rigs faster (yes, it switches faster than browse mode) and assigning it to a sequential PC.
It gives a nice way to also keep the stock rigs on the device, unmodified in the browse bank, and, per performance mode slot, you can make whatever changes you want (the same rig can exist, with individual changes, in any slot).
I'd actually disagree with you in saying that most people use browse mode. People that don't use it are often those that haven't tried it and haven't seen the benefits of it. Even without a controller, browse mode feels clunky to me. Performance mode is visually laid out and displayed - you're not trying to guess or blow through menus, at least after you get it set up in a way that works for you. And, using RM now with performance mode makes setting it up a breeze.