I'd like to know too!
How did you get on with mv6 towing? reckon another 30mm would be too much?
grumble grumble rub it in 
It was fine actually, a bit better than the CD which is better than the 2.0CD
30mm less than an MV6 - would be borderline for towing, ground clearance on the jockey wheel would be the issue
I've just measured heights on the estate with standard set up, with new shocks fitted, with a view to changing out the springs and it is not looking good for lowering.
Vauxhall quote a minimum height to the center of the ball of 350mm good so far as mine sits at 390mm so allows for 30 - 40mm of drop but the problem then arises that the caravan manufacturer recommends, usually meaning "we'll void your warranty if it's not in the range specified, between 380 and 450mm which leaves me 10mm.
Basically the choices available are standard length uprated rears and standard fronts or bite the bullet and try and get Irmscher I-line springs which give up to 20mm drop and hope there is a bit of sag already in the fitted springs.
Just phoned Vauxhall dealer while writting this and have been told that standard fronts are £143 + vat / pair and £193 + vat / pair for rears and a s et of I-lines are £250 + vat so I guess it may be getting lowered! Police springs are available on order but as they are dearer still I didn't ask for a confirmed price.
Steve