Millions of people have asked your questions before. But I'll answer anyways.

No, you don't need a new cat as the cat-back exhaust bolts to the cat. The larger diameter exhaust (which this probably is) will sacrifice low end power and free up high end.

Both ram and CAI sacrifice low end. Ram air is a little freer flowing at the high end, but also suffers from when the engine bay heats up (like sitting at a stop light). The CAI tend to also have a mid range hump increase around 4.5k RPMs, and then free up again at the high end. CAI are susceptible to hydrolocking your engine (aka damaging engine) if you drive through deep 8"+ or so of water.

I/H/E are never "big performance" gains, so don't expect it.

If you have a D16Y8, you'll never be fast without forced induction or an engine swap (or prohibitively expensive internal mods).

I have a theory that most people wanting I/H/E never redline their car. If you don't redline, you aren't using your engine to it's potential. Change your oil to synthetic, change your transmission oil, new spark plugs, new spark plug wires, do an injector cleaning.