Apologies to @RRdriver
for taking over your thread, but I've got more results to share. Hopefully helpful to someone out there:
At some point I thought I might have loose steering head bearings, but I checked them and they're fine.
I believe now that jittery feeling with the GP Suspension 20mm valve kit is just due to a lack of high-speed compression damping. It is more obvious at higher ambient temperatures (effectively lower fork oil viscosity) and can be mitigated by further closing the adjuster needles. A softer carcass tire---like the Pirelli Supercorsa SP that I ran at Buttonwillow (in mild weather) and on Day 1 at Chuckwalla (in hot weather) the following weekend---seems to be a better complement to this "looser" damping feel, perhaps because the tire itself will absorb more of the minor imperfections on the road/track. With a few minor tweaks (thanks to AJ at Paradigm Suspension) I was able to really find my flow that first afternoon at Chuckwalla. The track itself had a lot to do with it, I'm sure. It's fairly smooth and much easier to find the fast line compared to Chuckwalla.
By the end of Day 1, though, the Supercorsa SP's were done.. and cheap bastard that I am, did not take the opportunity to move up to the Supercorsa SC's. Instead I switched back to the much stiffer carcassed Dunlop Q3's, and spent another day chasing the right setup. Or perhaps re-calibrating my senses to the different feedback. We also changed directions to CCW, which was interesting. By the end of the day, though, AJ had the bike back in the sweet spot and the final session(s) were really enjoyable.
In the weeks since I've just been commuting, but I've made the geometry even steeper by taking the 8987 length/ride height adjuster to the limit.. and I love it. It seems to have helped settle the front end as well, by shifting some weight onto it. I've also noticed a clear improvement in damping feel when the temperature drops down below 20C. As such I would imagine this kit would perform even better in the typically hot weather at our California desert tracks with an oil that's about 5 points more viscous at 25C, i.e. 100% Red Line Medium. This would jive with Peter Verdone's findings with his 600RR. The GP valve kit (in this application, anyway) utilizes a compression stack on the mid-valve as in the stock setup, so it makes sense that it needs a thicker oil than typical.
Of course, having finally achieved near perfection I was presented with another upgrade opportunity I couldn't resist: another 8987 with upgraded "AMA kit" valving and a set of forks with GP Suspension 25mm cartridges installed. So the folly continues..