l have an Infiniti G35 with a skunk2 plenum spacer and a cold air intake. What is the next best step to get more horsepower? Pulstar plugs? Higher flow muffler ?Headers?

first you want your car to breathe better
intake and exhaust
then feel free to add heavy mod like turbo

A cat back exhaust system.

Cat-back exhaust, and then from there I'd go F/ l in the form of a Stillen Supercharger.

A cat back will not do jack but make it sound better if that what is what u want to hear. The choke point is the Cat, so what is good to have a larger exhaust on the rear? What u need is to increase the speed of the gases going out. A larger tube will slowdown the gases & will not purge like it should hindering performance.

Why is advertised? Is a multi-million dollar industry & does pay to advertise.

You want performance? Get in to race cars & u will clear a crap load of myths about these so call performance parts.

In order to get the desire effect, u need to test ur car in a Dyno. If is not done correctly, u will have a money pit for a car & will run worst than factory settings.

Dollar Per Dollar, a computer reprogram is the best. A tube with a piece of foam does not do jack! By the way? What that spacer does? l bet if u take it to a Dyno, u will see the same numbers as stock.

stillen supercharger