This is not my first Guitar Synth rodeo as I used to own a GK3 and a GR20.
So, the GK5 not only lacks the basic controls the GK3 has (vol, mag/hex/blend selector/preset next/previous etc), but it also lacks the radius adjustment the GK3 has.
Also, the manual lacks important information regarding the installation, that wasn't a problem for me since I already had the experience with the GK3 (two different ones, installed in several guitars), I really don't know what Boss were thinking...for a newcomer this can be a problem.
Now, the tracking IS better but I doubt that has to do with the hex pickup itself (GR20 vs GM800, the latter is a computational beast compared to the first one).
Qualitywise, not much has changed since the GK3 era, so nothing to comment.
Ditching the 13pin midi cable and going with this "stereo" cable is the biggest improvement in my humble opinion.
Last but not least, I think it's overpriced, pretty much like most of the Boss products.
PS: you will deal with tracking issues, it's the nature of the beast, but if set correctly, the GK5/G800 tracks significantly better than the GK3/GR20 combo. Better...not perfect...