I would be very cautious here. first let me say I am not a fan of lowering the car for the sake of it. you may want to find out the complete set up for the Assetto Corsa 3200 which had a lower ride height. It also had heavier ARB's and stiffer springs. There are 2 or 3 spring variations on these cars so be careful with springs.
Beyond all this caution, the answer to your question is yes the ride height is adjustable, and there are collars on the shocks to allow adjustment. If you go down this road you will probably need to have the corner weights checked and the full geometry done as these cars are very sensitive to set up.
If they are the same in design as the 4200 ( & I very much think they are ), adjusting the spring seat position to lower the car is likely only possible with the struts removed from the car, or such is the common wisdom on this issue...
Make sure the threads on the damper are nice and clean before you try turning the castellated lock rings....
I have had my 3200GT lowered (15mm - Assetto Corsa spec) but it was as part of a complete AC exercise as suggested by Davy where I swapped out the springs for the AC items (surprisingly half the price of regular springs), an AC steering rack, the SM ARB and finally a full precision geometry set up but to AC specs which are slightly different to a standard 3200GT. As Tim suggests, it was a matter of having to remove the struts to move the collars but as I was changing the springs they had to come out anyway.
What I did find was that when the car was lowered, the AC springs fitted along with the AC rack but I was still waiting for the uprated ARB which was delayed because of problems with the bushes, the handling was a bit of a nightmare. Once the AC ARB was eventually fitted it was absolutely brilliant - I suspect the problem was the stiffer springs rather than the lowering but obviously I can't be certain, the rack probably wasn't a factor in this temporary mismatch type problem. In this half way state for a couple of months, the ride and the roadholding can only be described as "choppy", it took the uprated ARB to bring it all back under control.
yes I should have mentioned the struts really need removed and springs fully depressed to make adjustments and those adjuster rings need treated with care I have one that is seized in my war chest. REmoving the struts is not too bad really, although as I say corner weights and the need to remove and refit the struts makes this a long pain of a job.