1. NOS doesn't rate their nitrous kits at the wheels, only NX does. The way the do that is by running bigger jets. If you ran the same size jets in the zex kit you would see close to the same power gain.
2. The zex kit is great for under a 75 shot. Over a 75 shot, you'll need to go direct port, and have fuel/ignition upgrades.
3. You don't need an adjustable FPR, I used the zex kit with the motor totally stock and a 75 shot.
4. A 55 shot isn't necessarily 35 at the wheels...if you factor in 20% drivetrain loss you'd have 45 hp at the wheels....we don't lose that much power through the drivetrain.
5. You'll need a nitrous pressure gauge so you can monitor bottle pressure. Optimum is 900-1050 psi, anything under 900 you won't really feel....higher pressure, more power.
6. Go with NGK BKR7E plugs as stated early, gapped to .030" for nitrous use. Make sure your ignition timing is retarded according to shot size...at 55 hp you're fine at stock 16°.
7. I personally, and many other I know using nitrous think the zex kit is great, especially for the newbie to nitrous. It's an easy install, easy to use, easy to tune. It regulates the fuel pressure automatically according to the pressure of the nitrous in the bottle, the NOS kit does not. This prevents extreme lean conditions that will kill a motor on nitrous. Say you are running at 1050 psi, the Zex kit will add a few more PSI of fuel, where the NOS kit does not.
8. A window switch is a great idea. Set it a few hundred rpm before your rev limiter. If you hit the rev limiter, it cuts fuel. However, you are still at WOT, so the nitrous is still flowing...Nitrous + no fuel = kaboom.

Any other questions feel free to ask.