After sitting in customs for a week my Summit shipment finally arrived. Last night I fitted the new 300lb "high travel" springs from QA1. Pictured below against the old 250lb spring, they have roughly 1.5 fewer coils over 12".
When I removed the old spring it had sagged about 3mm, which I suppose is OK.
WOW what a difference the extra 50lb makes. I was able to unwind about 20mm of pre-load on the coilover and it's still firm on the road but with more travel before rebounding (damper settings unchanged) and far less jittery.
I also fitted a set of needle bearings under the springs. Makes adjustment a lot easier once they're in the car and less prone to binding.
