Streetporting (Sp) is an internal intake/exhaust modification that can only be performed while the engine is disassembled. It is similar to swapping cams on a piston engine, it affects airflow (think lift) and timing (think duration). SP’ing is to be regarded as any other airflow mod (like air filter or exhaust mod) as far as power increases go. A large Sp on a stock car, for instance, might not be as noticeable as a small Sp on a modified car. Sp’ing doesn’t really change your idle ability or rpm, though a large SP can change idle tone a bit and perhaps even make the idle lumpy. IT does affect your driveability, mainly your low end pull. Especially, for instance, pulling out on a steep hill from a dead stop. The gains on top end are definitely worth the risk, however. Gains from an Sp on a nonturbo are not large, but still worthwhile, around 5-15% depending on mods. Do not plan on passing emissions sniff tests with a port, though it still may be possible with effective emissions tuning.
I generally keep my ports fairly conservative, which is good for a street car. Exhaust ports are opened slightly earlier but kept mild for low noise and emissions. Intake ports are opened, within reason, as much as other ports, boundaries, and water jackets allow.