I've tracked my Miatas for years and never had a club require anything more than the broomstick test. Have you actually been turned away by a club?
I won't be paying anything in labor. That's what a garage and air tools are for.
The BK bar doesn't appear to add any additional impact protection... it just raises the level of the bar, and I pass the broomstick test just fine already.
I love those GT3 seats, but in all honesty, the stock seats hold me really well. They are 100% better than any stock seat I've ever seen. I still use the "lock" trick where I slide the seat back, lock the belt, and then push the seat into the locked belt. However, in one session, I forgot to lock the belts and it wasn't until I pulled in that I even noticed. Genuine lateral support rocks! Worst case scenario, I have an Ultrashield from one of my race cars that I could throw in there.
The stock shocks performed flawlessly this weekend. If they don't handle the stiffer springs very well, I could always rebuild them. They are after all, bilsteins.