I currently have 10-46 on my Strandberg. I’m considering going to 9s because I think I’ll have a better command of bends/vibratos with a lighter gauge. My concern is the whammy bar. Are 9s more likely to come out of tune with a whammy bar, in comparison to 10s? What effect does string gauge have on whammy bar use?
If it’s not stable, take it to your luthier; the guitar is a merely a tool, you decide about string gauges! I suppose in the worst case you can configure it to be dive-only. I have 8’s and my whammy bars seem to be perfectly reasonable, but I have nothing to compare them to.
The heavier the strings, the more tension is need at the claw springs. Normally this is a simple adjustment of the screws that anchor the claw to the body. In extreme cases, you might need to add springs to gain tension. It’s also possible to replace the existing springs with “stiffer” ones.
In theory heavier strings and higher claw spring tension might result in slightly more stable tuning due to the increased force required to move the bridge off of equilibrium, but for practical guitar designs and string gauges, the difference is probably so small as to not be worth worrying about. This also means the bar will feel slightly “stiffer” with heavier strings.
If you make sure all the points of contact of the string and metal are lubricated, then you should be ok.
I use Music Nomad Tune It, or Schumacher moly grease - this is for radio controlled cars, but it works a charm on guitars too.
I dropped from 10-46 to 9-46 on my FR equipped guitar and didn’t notice any tuning stability issues. I did notice that bends are easier, I enjoy playing it more, and I’m sticking with 9s for the future.
I also noticed some manufacturers and professional guitarists actually drop down from 10s to 9s on their whammy bar equipped guitars. For example, Schecter and ESP have some guitars that come in both hardtail and FR varieties and come stock with 10s on the hardtails and 9s on the FRs, and Tom Morello uses 10s on his Tele but 9s on his FR guitars.