I did the same thing. Had a nice shiny car in the driveway but lived paycheck to paycheck. In hindsight I wish I had kept my previous vehicle that was paid off (or, if I hadn't had a vehicle at the time, bought something much cheaper.)
I can't tell you not to buy the FR-S. You'll do what you want to do in the end and it seems like your mind is made up already. But I'd strongly advise not making the typical youth mistakes, and make sure you have enough money to put aside and save once your expenses are paid. And while we're talking financial sense and youth, don't go racking up a bunch of credit debt either modifying your new car. They're tempting and look like free money, but that's a hole that's much easier to dig yourself into than out of.
Living paycheck to paycheck sucks. Been there, done that. I'm finally out of that slump and won't go back willingly ever again. I do want this car bad. Like
really bad. I am so sick of driving uninspired slushbox four door family hauler machines (even if, admittedly, taking the Camry through the mountains is still a nice drive.) But I've also an understanding that I'm older and have responsibilities and priorities as a father and husband that push this to the back seat for now. I just wish I had saved back then so I could have my instant gratification now.
If you want something fun and sporty I'd highly encourage you to go look at a cheap used Miata. I know, stereotypes and whatnot, whatever.. But the only reason I don't own one right now is because I can't justify buying a vehicle without a back seat now that I have a child. That and I always feel a bit tall inside of them.
tl;dr: Do what you want, but make sure you're saving enough money each month as well.