depends on how restrictive the exhaust is...

the principle behind it is that backpressure from a restrictive exhaust impedes exhaust flow, and hence, can rob SOME hp at top end, when your engine is pushing out lots of exhaust.

there is also the approach that a little backpressure at low-midrange RPM helps pull exhaust out of the cylinders, letting more unburnt air come in (called scavenging).

however, on a N/A B18A, 60-70mm piping should be a pretty nice balance for optimal performance... so, if u have an exhaust on there already, provided its around a 2-2.5" pipe, you should be fine. if its stock, you will want to go bigger, and if its open, you will get a ticket driving there

bottom line is: open header = most top end power, but usually a slight loss in low-mid range response. but a very restrictive exhaust will usually give you about the most low end torque you can get, at the expense of top end power (and keep in mind the gains and losses here are very minimal)