In my 30+ years of woodworking (20+CNC), I've never ran into issues with wood density causing drastic (or even noticeable) rpm changes.
I have one of the very first pre-release Super PIDs.
Stock router speed controls often have very slow response times, so RPM can drop considerably when starting a cut, or if feed isn't consistent.
The Super PID is much more responsive. When you start a cut, it will still slow down briefly, but the SuperPID will correct in probably less than a second. RPM under load may be a few hundred rpm off from no load, but should stay in a much narrower range than most stock speed controls. But if feedrade is not constant, rpm will jump up and down as the load varies, by probably 100-300rpm.
Imo, the main benefits of the SuperPID, are slower usable speeds, and computer speed control. More consistent RPM is an added bonus.
With the low cost of chinese spindles and VFD's these days, It's hard to recommend a SuperPID now. You can get a 2.2Kw spindle and VFD for less money than a SuperPID and 2-1/4HP router, and the spindle will be quieter, more powerful, and will likely last longer.
But if you already have a router, and want better speed control, the SuperPID is a really nice upgrade. If you don't mind the cost.