Well, it's probably perfectly safe for the PS2, since we are probably not adjusting the oscillator (Not overclocking it). That would be undesirable .
The PS2 was designed to be highly reprogrammable, so I suppose that bumping the pixel clock rates via software control of the GS registers is still within it's operational parameters.
(The VESA 1280x1024 @ 75Hz mode runs the clock at about 135Hz, so it's not too much lower anyway)
If it was being overclocked, I expect to see some instability resulting from overheating after a few hours of gaming.
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