The "20k light speed" computation based on Wally's claim that they covered "every inch of the planet" contradicts what was shown on panel. As was shown on panel, they caused major amounts of collateral damage in each place they passed as they traveled across the planet. If it was never indicated on-panel that they caused damage that kinda damage on "every inch of the planet", then we need to chalk up what he said to character hyperbole.
Closest thing we have to what could be quantifiable there was when he stated (on the same panel) that "they circled the planet over a dozen times". In which case he was moving at no less than 1.7 times lightspeed.