No, 5mm extra preload won’t affect static sag that much, as it’s not changing the spring rate, just moving the “start” point in the load/travel curve. You can experiment with this yourself by just leaving the top cap unscrewed by about 5mm or until you feel you’ve no pressure on the spring………just to test, obviously.
What oil level/quantity are you using in the spring side, or the damping side for that matter and what viscosity?
They do improve with use, in my experience, and also benefit from lubing the seals and stanchions with Molykote 33. I must like them well enough anyway, as I’ve recently bought a second set – just to future proof myself, I suppose.
They don’t feel as good just playing about testing them as they do when you just forget them and ride – some forks are far more impressive in the “carpark test”.
They’re not, in my opinion, as plush as a set of 160mm 55 TST2 forks that I use out in Greece – I converted those from port orifice to shim stack damping and now they’re like butter but I don’t think they’re as controlled as the RC3 damper through rock garden type stuff, At lower speeds though I love them.
If you do shorten the spring you need to heat up the last coil and flatten it, before grinding the end square otherwise the spring will load with a curve over its length and rub the inside of the stanchion – I’d just leave it as is, to be honest.