To follow up on this thread, exactly one year after my original post I took the PADI Deep Diver course. As I think I said in my original post, my goal was mainly to gain confidence at depths of 100+ feet, as the narc feeling plus lack of experience tends to give me anxiety. I went with my idea of taking the course in North Carolina because the diving there is different from the tropical/semi-tropical diving I typically do, and I thought the change of scenery might challenge me.
As it turns out, my intuition was good, and although the course seemed just as devoid of meaty content as people had cautioned me it was, I felt it was a good choice for me. The classroom portion taught almost nothing that shouldn't already be part of a diver's skill set and routine, with gems along the lines of: keep your gear well maintained, stay with your buddy, watch and follow your computer, don't go deeper than appropriate for the circumstances, adjust your buoyancy frequently, etc. But it also reviewed some things that can't hurt for relatively inexperienced divers to review once in a while, such as DCS and narcosis. Unfortunately, the course glazed over some of the things that experienced divers have said should be part of every Deep course, such as gas management, emergency decompression, advanced buoyancy and trim, appropriate lighting, etc. We touched on some of these, but didn't get into them in any useful depth (pun intended).
However, the saving graces of the course for me were my instructor and the boat dives. My instructor helped me configure and adjust my BCD, which is something I had put off doing for fear I'd screw it up. We replaced some plastic D-rings with metal ones, and stowed by SMB in a way that was better than I had it. My instructor was a serious tech diver--trimix and all that. And the diving and my fellow divers on the boat were of a caliber unlike what I was accustomed to. The dive op took its Nitrox procedures seriously, unlike some lax procedures I've seen at tropical resorts, so it was a good reminder that casual/sloppy O2 measurement and tank marking is not a good idea. My fellow divers were mainly experienced wreck divers, and I saw every gear configuration I could imagine: BP/W, doubles, sidemount, rebreather, etc. I saw only a couple of traditional jacket-style BCDs, which is pretty much all I ever see where I typically dive. Everyone seemed to have way more gas than they'd need for these dives, in stark contrast to places I've been where surfacing with under 500 PSI is shrugged off by the divemasters as no big deal. Many divers had pony bottles, and I don't mean Spare Air canisters. I learned a bit just by talking with some of the divers about their gear and watching them. The boat had a good reference line system for descents/ascents with hang tanks on the reference line at 15 feet, which is not the kind of setup I get to use often, so it was good experience. Our first dive was with a group of AOW students doing their deep dive, so that was a good introductory dive and refresher for me. For subsequent dives, I had told my instructor I was interested in learning about pony bottles, and so he helped me sling one on my BCD. It was actually a 50% O2 deco bottle and larger than what I might get for myself as a pony, but the experience of carrying it during a dive felt useful. He showed me how tech divers switch to their deco bottles, and on our 15' safety stop I got to try out the procedure. (Who knows--maybe Intro To Tech is in my future.) I had also requested he show me how to properly deploy my SMB, which is something I have been wanting to learn, but the boat captain nixed the idea because it could cause confusion, with so many divers in the water and conditions being less than ideal. To local divers, I'm sure the conditions seemed fine, but for me the sea was a bit rough, the visibility low (30-40 feet), and the temperature low (73F below the thermocline on one dive). So all of this helped fulfill my goal of challenging myself in unfamiliar conditions. I don't think I would have gotten as much out of the course if I had taken it at some tropical dive resort.
I'll be in the Keys in a few weeks and hope to practice my skills on the Spiegel Grove wreck (which I've dived only once before, maybe five years ago, and did not feel comfortable with). Gaining confidence with dives in the 100-130 foot range is just a matter of doing dives like these every so often, preferably in the company of divers of this caliber. I'll probably buy a pony bottle. And at some point I'll practice deploying my SMB. If nothing else, the Deep Diver course showed me that maybe all that easy warm-water resort diving I had been doing was making me complacent.