Well, 2.25 inch is the common size in Japanese exhaust sets, so no problem with that even if you go 2.3 inches. What kills power is probably the bends and kinks if it's not mandrel bent.
VTEC not much modding required, unless you go turbo.. The stock one output is also good already. and any mods after that will sacrifice fuel efficiency and noise for power.
Basic mods is always the intake, open pod like yours is okay, and exhaust set including the extractor. If you are having white tips on your plugs, get VAFC or fuel regulator for a quick and/or dirty tune of the fuel maps.
Other things make such a marginal difference, I don't think it's too much of a concern, just remember that some parts are 'wear and tear' items (plug wires don't last forever, nor do oil seals, distributor internals (igniter) etc. etc.
Amazingly, I'm still running with the same distributor after 240,000 kms +.. Now I'm getting worried it mamposes suddenly.
