I had the 2006 zx10r shock on my sv and I'm 12 stone. Wasn't over sprung IMO and no cutting required (although I gather you've already done this to accommodate your existing shock). Its spring rate is only 8.3 kg/mm as opposed to the 9.2 kg/mm of your existing zx6 shock which I agree is over sprung for your weight.
Bibio is right about the gsxr 600 k4/k5 being a a lower spring rate but it's actually less than the standard shock on the sv (stock sv is 7.7 kg/mm, gsxr k4-5 is 7.6 kg/mm). Do you really want to be going softer than the original rear shock at 12 stone?
Check out this chart to see what I'm talking about
https://docs.google.com/spreadsheet/...COvG24gH#gid=1