The Famous "M&M" act isn't really applicable. If you modify the vehicle beyond factory performance levels you are techically "abusing" the vehicle and they don't cover you in that case. Activities like racing, sled pulling, lift kits, monster tires, ect are seen as stressing the vehicle beyond design parameters and the OEM will VOID your warranty no questions asked.
Is it a "grey area"? Is it possible you may get things warranted in spite of it? Maybe. But, do you have the ten's to hundred's of thousands of dollars in legal fees to fight it for years? Best advice is to never take a modified vehicle to a dealer and expect warranty service.
If you want the warranty with no issues or surprises, don't mod until the warranty period is out. If you can't wait, don't expect "issue free" warranty coverage.
Remember the old adage, "If you want to play, you have to pay". There have been cases where people have had dealer known mods done and a factory rep inspects the vehicle and voids it out. Remember, the dealer is NOT the one backing the warranty, the factory is. Here's an example. Chevy won't even diagnose transmission issues if you have non-factory sized tires. I'm sure the other companies are similar.