Oh yeah sorry didn't realize how new you were. Stock Civic SI? what year?
The midpipe is the piece of the exhaust between the header and the cat-back. "catted" = with a high flow catalytic converter so you aren't screwing up the beauty of nature by going cat-less
VTCS is a tumble control mechanism in our intake manifolds designed to reduce cold start emissions. It's butterfly valves chillin in the intake manifold though blocking airflow, pretty annoying. It isn't a simple bolt-on install though if you haven't ripped off an intake manifold before.
Underdrive pulley replaces the pulley on the end of the crank that drives all the belts. It is lightweight with a slightly smaller diameter and underdrives all the accessories (alternator, power steering, a/c pump, etc) to reduce load on the engine. It sounds like it could cause problems and on some cars they do. On our cars, no issue really. I've had mine for probably 30,000 miles now no issue. You'll need an impact wrench to do the install though.. other than that it's pretty easy.
The intake cam can be done without removing the crank pulley in theory but I had to get the pulley off so it was a bit more of a process (needing to retime the engine, etc). Re-timing the engine isn't anything to worry too much about with these cars because they have noninterference engine.
The MP3 ECU is the ecu Mazda made for the Protege MP3. It advances the timing, which is what gave the MP3 140hp instead of the 130hp the Protege5 has. It requires premium fuel to get the gains. If you use a lower fuel grade it just retards timing back to the what the Protege5 has basically.
Things not to waste your money on with these cars include:
1. bosche spark plugs.. they cause misfires. Just stick with the cheapo (8 bucks or so for the set) NGK copper plugs, either the stock BKR5F-11 or the JDM-spec long reach plugs, the ZFR5F-11.
2. Spark plug wires -- oem for the win. or NGK, which I think the OEM ones were anyway.
I was able to pull pretty nicely on 99 Civic SI with the mods listed. Civic had a catback and intake.
If you need any other info let me know.. I'm on here way too much so even stuff that I don't have experience with I've gathered knowledge on