Had it on the car for a while. There is a slight rattle at higher RPM's but I don't think it's coming from the shifter.
After hearing the rattle, I pulled the IRP shifter out and took it almost entirely apart (the only thing I wasn't able to get off was the button - more on that below). Greased everything (grub screws, bolts, bearings, etc) and put the OEM shifter back in so I could still drive the car in the meantime. Still rattles with the OEM shifter at high RPM's, I believe it's coming from my linkage. Crawled underneath the car and shook the linkage by hand and that seems to be it.
The button sits on a bolt that threads into the aluminum body of the shifter. There is a grub screw holding this bolt down to prevent it from vibrating loose. Seems like this grub screw is over-tightened from the factory, because it bent the threads on the bolt that holds the button. I was able to rotate the button a few degrees by hand but with the bent threads, it'll never fully come out. From what I've seen on the FRS forums, the newer revisions of the IRP shifter has an o-ring around the button so I don't think that'll be a source of the rattle.
Putting the IRP shifter back together, I accidentally over-tightened the grub screw that holds the reverse lockout cable in place. This caused the end of that cable to push into the aluminum lever of the shifter and dent it, preventing the reverse lockout button from going down. It was a nice $177 mistake on my part
luckily IRP was able to send me a replacement lever.
Overall, still happy with the shifter. Compared to the Coolerworx, everything just seems to be tighter in terms of tolerances and I prefer the reverse lockout cable being hidden. The piece that redondoaveb posted (replacement Coolerworx part) is a lot more snug on the IRP compared to the Coolerworx - mine had a bit of play when I took the Coolerworx shifter out.
Photo attached, don't over-tighten those 2.5mm and 3mm grub screws if you decide to take the shifter apart!