I got a way worse grade than my classmates for equal performance. What should I do? The professor for this course was, according to every one of my classmates (only 7 of us total so I could get all the opinions) a nightmare. To illustrate, in the last week of class, after we present a final assignment (no instructions other than the title of the assignment in the syllabus), he asks us if we'd been getting any of his emails for the whole semester. We said no. That last class was a sizable part of the grade but obviously no one had instructions or any knowledge of the percentage of the assignment relative to our grade.

Each week people in our course were confused about material and procedure and we all thought we were fools that should have known better. But we all had realized, in this last class meeting, that there was a reason for the disconnect: the instructions were not going through to us via email.

There was a discrepancy in my grade vs the other classmates grades, even though there is no discernible performance gap. I got the lower grade. The classmates I spoke with were surprised, for the same reason I am - namely they did not perceive a noticeable performance gap. My skin is crawling at the thought of asking the professor what the reason for this low grade was, but should I do it? Or can I go elsewhere (i.e. a Dean or something) first? I know for a fact other students are willing to attest to this professors disorganization and other bad qualities (some already have). I also suspect this professor has had complaints lodged against him in the past for similar reasons.
Easy answer:  Go talk to your professor at office hours.  You should escalate to your department chair or the dean only after you've tried in good faith and failed to resolve the problem directly with your instructor.  Don't escalate unless it's genuinely serious.

Try to be open-minded toward the possibility that there were deficiencies in your work that you may not have spotted that explain why your grade was lower than you think it should be.  Your fellow students' opinions are not reliable indicators of anything nor is it helpful to cite their complaints or opinions as justification for yours.  Go with the expectation you will learn something, not with the sole goal of getting more points.