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Enquired about a custom tune at the same time as service, and they're also recommending this £125 (!) piston for the damper. It's said to increase flow under both compression and (most notably) rebound, for a wider range of tuning.
I've found a few older threads on here where people had it and it gave "polish" to some older model Fox dampers, but I'm interested in why/how it works and when is it a valid candidate for upgrade. Obviously if it's fitted along with a getting a custom tune it wouldn't be possible to attribute any improvement to either individually. Regardless, I'm unlikely to buy it, but we can nerd about suspension anyway.
With some basic knowledge of suspension and dampers, I can see how this may be a useful thing if you're running your compression or rebound damping fully open but still have too much damping - i.e. you're at the edge of the adjustment range and are flow constrained by a piston with too small holes in it, like this unknown Fox one on the left:
My compression damping is indeed fully open and could probably do with being softer, but my rebound is in the middle - so I've already chosen to not have it as fast as possible. My fork is a 2020 Pike, which I believe has a piston something like the unknown RockShox one on the left:
The difference in total port area is less obvious, and might actually be negligible. I understand half the holes are for compression flow, and the other half for rebound flow, and the sizing being a matter of tuning.
What's STW suspension folks' take on this? Is it only useful if you have a piston with tiny holes and your fork is still too damped with the dials all the way open?
I had one on a 2014/2015 pike. It was very much a polishing exercise. It turned out the luftkappe modification of the air spring made a great deal more difference.
Like you said, difficult to isolate if a custom shim stack or the piston would make more difference. Give the minimal port size increase, my money is on the most benefit coming from the custom shim stack.