Cheers guys.
GTSboy, the car is currently sitting at roughly 345mm both front and rear. I actually had a set of grooves machined to my specification to give the car an extra ~5mm of forward rake. I was thinking of going an extra 10mm lower to 335 front and rear.
djr, I'll have a look today, but you're both probably right, 20mm lower will be excessive.
I'm still interested in my original question though, does operating a new damper in the final few inches of its travel cause damage to the valve stack, piston or any other components?