Well, you have a few options here. If you're obtaining 8 psi within .5 sec with the nos, I would think heavily on going with a larger turbine housing or turbine wheel/housing combination. That would help make boost control easier, and would help your top end performance.
2 step rev control would only be necessary if 1) you're looking for very consistent launches for bracket racing, 2) your t/c stall speed, when on the nitrous, is too high, 3) you're unable to control the boost level during staging to control launch rpm that way.
For example, my nitrous hit size and t/c stall combination is such that, when on the nitrous, my t/c does the rev control for me.
You can use the nos hit size to fine tune the launch attitude of the car. The nitrous hit size, hot side (turbine wheel/housing) size, and the T/C stall (on the nitrous), tuned together can be a very good tool for coming up with the launch attitude you're seeking, making a 2 step rev control completely unnecessary.
Let me put it another way. If the hot side, t/c stall, and the nos hit size are poorly matched, making boost control at the launch difficult, then a 2 step rev control would most likely be necessary as a band aid method of controlling launch rpm.
If you do use a 2-step, only use timing retard if the nos is going to still be active during the 2-step. Do not drop ignition or fueling to cylinders where the nos is still on. Otherwise, just make sure there is no chance that the nitrous can come on during the 2-step if you plan to drop ignition and/or fueling.