I'm around your age and have seen several urologists over the years. (My issues were mostly about feeling like I was going too often and not being totally full/empty.)
I think you can pretty much plan on:
1)Providing a urine sample when you first go in
2)The usual dialog of symptoms and medical history
3)A genital/hernia exam and a rectal exam.
Based on the symptoms you described, I'd say there is a fairly good chance that they will have you pee into the flow test funnel for your initial sample and then do an ultrasound to see how well you've emptied. I've never had them do one on me pre-voiding. For most of my urologists, the nurse does any ultrasounds not the doctor, so if you like the idea of being undressed in front of the nurse, just lower your pants a few inches below your genitals unless they are very specific about not needing to do it. (I think they can do it without you really lowering your pants at all.)
I think a Cysto is a real possibility, but unless they are very slow that day, there's a good chance they'd do it in a follow up appointment that you'd have to schedule when you leave. For a "standard" cysto, I've never found them terribly painful or anything, just a little uncomfortable when going past the sphinctors or prostate. If the reason for your slowing is some sort of stricture (scar tissue of sorts) in the urethra, then it might be a little more painful. Anything more than a standard cysto with a bigger scope or anything they would likely do as an outpatient procedure in a day surgery unit.
If they don't find anything specific with cysto, etc. they may recommend urodynamic testing (you can google it to get a description of what they do). I really enjoyed this because I had a nice female technician.
As far as the piercings go, anything that would draw attention to your genitals likely will be noticed, but there's really no way to predict the reaction since it will vary depending on their personality. I trim my pubic hair very short, and the tech actually commented on that because her experiences were that removing the tape from the catheter was more painful for people with lots of pubic hair. So I'd expect that if you leave your piercings in for the exam, they surely will notice and the doctor at least would most likely ask some questions about when they were done, etc. just to make sure he doesn't think they have some impact. For example, even if not through the urethra, if they could potentially compress the urethra or if there is some scar tissue built up around the piercings, etc. If you take them out, you probably should still tell the doctor you have them, especially since the holes may be visible and would seem odd without the explanation.