i was looking at them yesterday..... the tyres have about 10,000 left on them at a guess...... at this point, 20's are out of the question.... my son just got put off work and things aren't too bright up here in the Hunter Valley, so i don't want to spend too much just yet.
Looking at them, they look like they could handle an extra 10mm per side..... i did a search but there are no threads on what people have put onto stock rims.
As disorganise said, increase sidewall flex will be a result depending on what brand/style tyres you get. The tyres fitted currently are a perfect fit to the rim (in terms of maximum rim width to tyre width ratio). The 10mm wont be noticable visually (ie will probably look the same) but structurally the steel belts will not form the most rigid design causing poor driving characteristics (poor change of direction response) with the possibility of the tyre rolling off the rim (though it mainly happens to tyres with taller profiles). If for the rear only, you probably wont notice it but its best to stick with stock dimensions unless you get wider rims.
Cheaper tyres have weak sidewalls. Having used tyres such as federal 535's (common sports tyres) on the track, I have found directional transitions to be very sloppy. Usually bridgestone tyres (potenza series) have nice strong sidewalls but its a matter of how much you want to pay.
My opinion best off just leaving standard profile, but you could get away with a 255.