After lots of research from members here and there...
The only real mods worth any money are lightened flywheel and chip. Those will net you some small gains, particularly midrange.
Cold air intake... B/S. Not very cold. Too much underhood heat unless you close it off from the bay. If you do, you will see some very minimal gains. Put in a clean filter, does not have to be K&N, ours are of good size for our motor and cut a small hole in the front of the box, does as much good as anything else pretty much.
Better flowing muffler... A cheap $50 muffler is all that is needed. Worth a couple HP. Catback has been shown to be no better than just a cheap better flowing muffler.
Coil On plug... not really much power, but it will run better and smoother, and it is cheaper than good plugs. Better yet, should last longer. Win-Win situation, but no real power bonus.
Injectors, not really much help unless you have done the rest above.
All of this is good for only a few peak horsepower.
It does however give you a faster car with a better midrange.
All of this has been well documented over the years.
From what I have seen, the power in our motors is locked up in the heads. Fix that and now the cams are a problem. Look at what Metric Mechanic does to a motor. They are not using a bunch of new parts.