adjusting the spring preload on the rear spring will not do a whole lot to your height but do more to the stiffness....making it softer...you can shave some of the padding in your seat, wear boots or shoes with a thicker sole or lower the bike with lowering links, which will reduce ground clearance and maneuverability.
Suspension is something very important with sportsbikes...so whatever you do becareful if its something you are not sure about.
Keep in mind anything you do to the rear, you will have to harmonize with the front, simply changing the ride height on the rear will affect your handling greatly. Its best if you dont know what you are doing to take it to a suspension shop and have them fine tune it for your height/weight.
if you tighten it it should take out some of the droop if im not mistaken, making it taller, and if yo ukeep going it will only hold the shock all the way out and not want to compress. the looser the spring the more droop and the lower the bike will sit and the softer it will ride. you would need a physically shorter springto lower it as compressing a spring of the same rate will only make it stiffer or move the shock further out of its travel