I've done some skiing in Michigan when I went to college at UofM and did a couple of NASTAR races. If you're in the LP (Mt. Holly, Pine Knob, some other random hill), there is absolutely nothing there that will require the use of GS skis. You will struggle to build any speed at all, making it very hard to turn on GS skis.
There is perhaps 1 run in the UP (Boyne) that "may" require GS skis, but even that tends to be a joke given how short it is. In my opinion, the trails in Michigan are far too flat, and where there is some steepness, far too short to build up the speeds necessary to adequately flex a proper GS ski. Short, precise turns are what you will need to master and SL skis will be more than up for the task.
With that said 165cm is probably too short - I would aim for 170cm -175cm at your height and weight and even then wouldn't go for the super stiff SL skis given you're going to have a hard time bending them at your weight and given how slow the Michigan runs are. Given how you mentioned you are struggling on turns, think a few lessons will be in order. Going with shorter skis will make things easier but will not encourage you fixing flaws in your technique. 155cm is far too short for you, especially given there is nothing challenging enough in Michigan to make you struggle with 165cm or longer skis.
Btw, what high school are you racing for? I had a few college buddies who went to East Jordan in the UP and the Bloomfield Hills schools in the LP, and some of them owned GS skis, but some of them not - but then this was back in in 1990s and ski technology has obviously changed a lot since then.
Edited by iheartnyc - 1/12/15 at 1:48am