one rule of thumb is that higher voltages will ruin your chip much faster than normal voltages.
getting to 5.5ghz on a SB needs a larger voltage increase than you need to get to 5, proportionally. ~ 1.5V
that kind of voltage definitely is not good for 24x7 usage.
that said, 4.7ghz is possible with very little voltage bump. that should be stable 24x7 for a long time.
off course, every chip is different.