If the paticular modification is found at fault for your problem, than your warranty is void for that paticular problem.
If adding an air intake causes (example) your check engine light to go on, because too much air is causing the car to run richer than normal, they can charge you $XXX.XX to turn off the light and performing a service check. If your suspension went bad, they can't go around and say it was caused by the air intake.
Changing spark plugs and wirings is falling in the grey area. Engine stumbles, engine noises, engine problems will give the dealer more ammunition to point that the spark plugs and wirings are causing the problem. You might want to avoid this modification as it might give you more headaches than the benefits it gives.
Strut bars may be a safer mod since the bar affects more of the body of the car than the power train. Of course, you may void certain parts of the chasis of the car, but it's not likely that your car will split in half anytime soon. This is probably a more conservative mod that wouldn't cause much warranty headaches versus the others you mentioned. BUT be aware, that the strut bar can avoid the warranty for any problems it causes (normally the body).
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