There are several things that you can mess up with bigger tires and rims. First with a taller or wider rim, you have to worry about the kingpin offset. Kingpin inclination is the the line that ball joints on the end of your control arms make when you draw a straight line through them. If you extend that imaginary line to ground level, then measure the distance to the centerline of the tire, that is the kingpin offset. For ideal performance this value should be zero. If you go with a taller tire, you can make the line go outside the centerline of the tire, causing a tendency to understeer, inside the centerline causes the tendency to oversteer. This also affects the scrub radius of the car. If the scrub radius and ackerman turning circle don't line up, your tires will squeel in the corners (reduction of the max 1g lateral friction threshhold).
Changing to higher spring rate tires also affects the ride and response times. Since the wheel and suspension spring are in series, the higher spring rate of the suspension spring dictates system performance more, but the tires also contribute.
If you go with big rims it also changes the height of the roll center (the imaginery line about which the car rotates in a corner). If you drop the car to accomodate for the change in ride height, the geometry of the suspension linkages is altered, is it now set in the jounce geometry and now is less capable of taking hits from the road.
The tire camber and caster may also be altered. Camber will be most likely positive since th tire will be in jounce, which is advantageous for track performance but negative for street performance, since the tires will wear unevenly and less of the tire surface is in complete contact with the road. The caster will only be altered if your suspension has self steering characteristics designed into its geometry. I do not know if this is the case with the Focus. Self steer is the tendency of the tire to move, relative the body line when the body of the car rolls relative to the ground in a corner, in the direction of the turn in order to provide a fater turn rate and tighter turn radius.
The good news is, unless you are racing, and putting your car in at the limit situations, you probably won't notice these differences unless you are a trained driver or go with HUGE rims. most suspensions are designed to allow a variance in wheel sizes within a given range. If there are any other chassis dynamics/suspension questions... I'm your man.
YES I do agree that is total BS.The svt suspension is designed for larger diameter wheels.
the only interference possible would be rubbing in the fender wells but buying correct offsets and staying with the factory ride heights you should be fine.I have seen a few SVTF's sporting 18'' rims.
NO RICE INSIDE,,,,,,,,