Stock I belive AZ ZBum tested it to be something like 5-6hp and with mods you'll gain more. Also, on the 300zx there is no "cold-air" intake, there is just intake. Cold-air means the filter is in a location to get cold air such as on hondas where they route the filter to behind the bumper. On the 300zx you can't really do that so it's just a regular intake.
I think at some point similar "cold-air" set ups were proved to not really be effective becuase for 2 points. The first being that the intake pipe is so long before it gets to the engine that any cold air that was sucked up is heated up while traveling in the pipe. The second point being that there is a coolant line that connects to the TB which also aids in heating any air that travels through the TB. So while the air may be cool hitting the filter by the time it gets to where it matters to make hp it's already heated up. So basicly all that seculsion of the filter isn't worth the time. Removing that bar like it states in figure 2 does help with letter more air get to the filter so I would do that but the other steps are pointless.
The problem you had where it would hold back...POP... and then go is not due to hot air going into the filter. That has to do with something completely different. Many people here run just a cone filter sucking in the underhood air with no side affects other than the 3hp gained. Like I said the cold air may be sucked up by the filter with the cold-air set up but by the time it gets in the intake manifold it's already hot so it doesn't make any difference.