This is a tongue-in-cheek, sarcastic yet truthful, look at basic performance "mods" for those who are staying stock-ish, or for those who are too cool for school
Fresh Oil & Filter
(BHP: Do You Have to Ask?)
Before you reach for that tune, exhaust or turbo kit, first ensure your engine is actually as daisy-fresh as it can be. This means you must be in the practice of changing the oil at the recommended intervals and with the recommended oils. If your engine is already gunky, if you see smoke, then you are losing power from that alone. Get your engine treated and then resume normal intervals. Otherwise, don't be surprised when your mod blows your engine and the dealer says you're SOL.
(WHP: Up To 100%+)
Some people might be surprised at how tires can directly affect a car's performance. Worn tires can be deceiving as they gradually become useless. If you just got a used car with worn tires, you cannot know anything about that car's handling or acceleration until you get fresh tires. Tires should be new enough to operate within their normal parameters of adhesion and water dispersal, otherwise, they're doing you no favors. Easy burnouts and four-wheel drifts are fun, but that's not how the car is supposed to handle.
(BHP: Easily 10+, from Low-Tier to Top-Tier)
You can seek performance with mods such as a tune or exhaust, but in some cases, that much or more power may already be hidden in your stock engine, masked by cheap gas. This can cause cloggy filters, dirty injectors, fouled plugs, and dirty valves, not associated with hi-performance. Some people will claim that "all gas is the same", but while all gas comes from the same pipeline sources, the advanced chemistry and additives from the best brands do not. So, if your stock engine is sputtering or sluggish, that's not Hyundai's fault, but yours. The simplest solution is to stop going to Skippy's Mini-Mart for your cheap gas. If you're proud of your car, gladly spend a few extra pennies on the best possible gas brand in your area, and even then only from the cleanest stations (which takes trial and error). Give it some time, say a few hundred miles or more at varying RPMs, once you've found a good station. The power you will regain might be all you need.
Remove the Carbon Filter
(Some US 3.8s)(BHP: Easily 15+, especially with gas-mod)
For some models, EPA regulations mean that there is a secondary air filter in your airbox, commonly called the "carbon filter". It is post-paper filter preventing fumes from coming back out of the airbox in the rare event of a backfire or sputter. Since you've done the above "mods", your engine will not be sputtering or backfiring, thus you do not need this extra filter, and removing it will not likely affect your warranty. It will affect your ECU tune, though, because the paper filter is a high-flow model very similar to a K&N in terms of flow rates (it has to be to counteract having the carbon filter in there), so removing that carbon filter gives you the equivalent mod of a non-carbon-filtered car going from the normal-flow paper to a K&N. Thus, the claims made about gains from a K&N are what you can expect from removing this carbon filter and using the stock hi-flow filter by itself. Just as with gasoline, the ECU will retune your engine after removing this filter, so give it a bit of time to adjust and settle.
(Non-Carbon-Filter Models)(BHP: Easily 15+, esp. with gas-mod)
If you do not have the "carbon filter", a simple mod is to replace your paper filter with a K&N hi-flow. This will also not likely affect your warranty. Just as with gasoline, the ECU will retune your engine after changing to a K&N, so give it a bit of time to adjust and settle.
Turn Off Traction & Stability Controls
At low speeds it might not seem to make a difference, but turning off the TCS/ESC system (10-12s) or TCS & ESC systems (13s+) "frees up" the power that will be lost when it kicks in (often when you least expect it). How much power is that? It depends on the Coupe's year and incident circumstances, but most of you reading this probably know how your car reacts to bumps, small slides, burnouts and 2nd gear pops when these systems engage. Without the systems monitoring wheel spin/yaw, and thus not applying damping, yaw controls, or engine cuts, you have the full engine power available at all times. Some people even believe there are other tuning parameters that are affected positively by turning TCS/ESC off (which may be apocryphal), but regardless, your car will certainly be faster and more powerful without these nannies - that is, in most controlled situations! In other driving situations, such as in foul weather, though, these systems could save your life!
The Obvious Conclusion
If you've done all these obvious and easy "mods", your engine will be running in tip-top form. In some cases, you may even find that you need no aftermarket mods! If so, following these "Captain Obvious" tips will mean your engine is running strong and wearing correctly, and you will also have no warranty issues if something were to come up with your dealer. Only then can you accurately judge whether you need further mods, or how those mods truly affect your car's performance.