Using 6x58W fluorescent lamps instead of 6x55W, what could possibly go wrong?
With no information about the fixture (not even a link?), it's really impossible to speculate.

However, based on the bulb you mention, and the fact that it uses an external ballast, I can say that probably nothing bad will happen.

It's the ballast that controls the current through a fluorescent lamp, and therefore its power consumption. So, since the ballast is part of the fixture, it's very likely that the fixture will simply operate the bulbs at 55W, causing nothing more than a 5% drop from their nominal light output.