Eat like a pig all winter and buy some 16" wheels for your baby
Erm.....seriously - I think the help of a suspension expert might be useful, I seem to remember a guy in Nottingham being recommended in a thread here ages ago.
I think changing the seat pad or having it rebuilt might be an easier / better option than trying to reduce the ride height by 15mm, certainly my Corbin seat is a good 10mm lower than my original Ducati one.
But - I dont know if any aftermarker seats are made for the SL.
Is the ride height correct anyway?
Good luck Colin !
Regards - Frank