They can't void your warranty for installing aftermarket plugs. If anything goes wrong with the car, the dealer has to PROVE that it was due to your modification. There's no reason spark plugs would hurt anything as long as you install them properly.

Platinum+4 plugs are a waste of money. They last a long time, but they hurt performance because the 4-electrode design obstructs the spread of the flame kernel in the cylinder. The best performing plugs are the non-platinum ones, but you'll have to change them often. The best compromise between performance and longevity is probably just a normal NGK or Bosch platinum plug. Even copper plugs should last you 30,000+ miles, though.

As far as actually installing the plugs, you'll have to remove the intake manifold, which is why the manual tells you not to do it. It's not hard, and is definitely worth a try.