Just from personal experience.
I had my PI, research advisor, a residency director and the physician I worked for (prof at med school) all read my PS and give suggestions.
I think that is really your best bet.
Get someone who writes a lot (grants, papers) and who preferably interviews candidates (PIs, physician, old professor). They really will give you the best and most concrete advice about this because they know how an application process works.
If you want, you might even wanna try essayedge.com (if you have the extra cash laying around).
I am in the application process now and I think one of the strongest parts of my app. was how much attention I paid to the writing.
The doc I used to work for (prof. at a med school), said there are three types of PS:
-the majority is so blah that they dont impact the app. at all.
-Some are so bad that they will hurt your app.
-Others (very few) will actually help your app and make you look more desirable.
My goal was to write a PS that would help my app to stand out in front of those who didn't pay that much attention to it.
You sound like you started early (I only started writing mine in june), so it sounds like you have it all set.
If you want, you can PM it to me, but I would still have one those mentioned above read it if you can.
hope it helps.