I went down the same route as you, I took the 3 day SB course last year at Watkins Glen, then took advantage of the discount for the 2 day advanced racing course which I took this year at Mont Tremblant.
First, the 3 day course is a basic, very basic course getting you into the car and teaching you some of the fundamentals. I to was excited about the SCCA lic and applied for it and got it. Just be aware you have to do a minimum of 2 SCCA races in one year to maintain your lic. I did one but couldn't get the 2nd one in. Lic is now expired even with the SB 2 day advanced racing course.
I know exactly what you mean about claustrophobic. Sitting in the car, all strapped down, can't breath and the heat is burning you up in the race suit and you can't move. I had to learn to relax and control my breathing before I panicked.
When you step up to the 2 day course, no more heel/toe, sequential shifter

Yesss. just blip the throttle and up or down shift. At this level you do some lead/follow with instructors but you are also encouraged to do some passing. This is where you find out who the fast students are. We had one student, an older man, at least older than me, keep blowing by me and I'm thinking, where in the heck is he getting all that extra power from? Later I noticed a SCCA National Champion patch on his racing suit, he was just taking a refresher course cause he hadn't driven in a couple of years..
The next level up for SB is computer lapping and racing. So if you want to do laps and get computerized info on your laps you have to go for the Skip Barber "lapping days"
One last thing to remember about the Skip Barber school is that once you reach the 2 day advanced racing course and above, you are responsible for damage done to the car up to a certain amount. Those spin outs can get costly. I spun going into turn 1 at Mont Tremblant coming off the front straight but didn't hit anything, thank God. Just dirt and grass flying everywhere.
All in all thought I feel I learned a lot after this last 2 day course that is very valuable, looking ahead, smoothness on brakes and throttle, but most importantly, thinking about braking and throttle while I'm doing it, whats the car doing when I do it and learning. I feel I was considerably faster by the end of the second and now more in tune with my CS when tracking.