The 2.3L motor has counter rotating balance shafts that are attached to the bottom of the block, kind of submerged in the oil pan and is run off a gear on the crankshaft. They act as a dampening device, removing any unwanted vibrations that may be felt by the driver inside the cabin of the car.
The benefits are that you would be removing over 20lbs of wieght from your car, and I think it was about 12lbs of rotating mass, causing parasitic drag on the motor. Your car would rev much easier both up and down. There is some hp/trq increases, some say about 5 each, but I think realistically maybe a 2-3 increase in hp & trq.
Warranty, well I would think if something was wrong with your car that warranted a tech to remove your oil pan and noticed the balance shafts gone. I think it would be pretty safe to say that your warranty would be null and void. I mean its not like bolting on a CAI or exhaust, your kind of messing around with the internals, & I don't think Ford would take kindly to that.
New Whip" '09 Scion xB "Release Series 6.0"
Old Whip" '04 ZX3, 2.3 Duratec, CD Silver w/ some stuff....