I got the 1.2 Sonic Springs installed and tried them out with a 230-mile ride in the N GA mountains Sat. The handling is MUCH improved, and this mod puts more emphasis on the S in the ST. The front end is now not the overly soft, mushy, sometimes unnerving response in curves that it used to be. Much more balanced and definitely firmer, but not excessively firm or harsh (for my taste). I was VERY pleased with the new handling.
My '06 ST only has 27K on it and was box-stock. The rear unit seems to be OK for now and no fork seals were leaking. I had first intended to just install the "drain tube" spacers described in other threads here. I even made the spacers from the "sink-drain tailpipe" as recommended but they look too thin and fragile. I made another set of spacers from 1" sched 40 PVC and devised a "no sweat" method to install them with a 6" gear puller. But then the Sonic Springs arrived and I decided to go with them.
After reading the instructions several times and measuring to make the spacers from the PVC supplied with the springs, it was apparent that the stock steel spacers were the correct length without additional spacers, so I decided to do a quick and easy spring replacement. I just put the bike on the center stand and did one fork tube at a time, no engine support needed. Pulled out the stock, progressive spring and measured the depth of the oil, sucked out all of the oil possible, and replaced it with 7.5 wt Maxima fork oil to the same depth. Then I dropped the Sonic spring in with the stock "bushing" and the stock steel tube spacer and replaced the cap.
It was a simple R&R except for the partial (80-90% ??) fluid change. Didn't have to remove any tupperware, the front wheel, loosen the triple clamps, or purge the air. This would be a 1-hour job the next time. $120 for the springs, $20 for too much fork oil, a torque wrench, a couple of end wrenches and Allen wrenches, and an old spray bottle pump. EZPZ.
The specs show the forks' full travel at 116.8mm. The sag with bike and rider measured 22.2mm, or 19% if my math is right.
The rear specs say 121mm full travel and the sag with bike and rider was 41.3mm or 33%.
So, the front is a little stiff and the rear is a little too soft, but the rear preload can be adjusted. Doing the static "saddle bounce", the compression and rebound dampening were very close being equal and optimal.
For anyone not needing to replace the fork seals but wanting to upgrade the handling substantially and easily this method would be an option.