Buy an impact driver (typically used on motorcycles). A few good whacks and I expect it will come loose without issue. There is no other way to remove the case screws on a motorcycle without butchering the fasteners. Similar application here.
http://en.wikipedia.org/wiki/Impact_driver
Of course these days they are available as cordless power tools as well...
http://www.ridgid.com/Tools/R82320-Impact-Driver/index.htm
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2001 Lapis/Black/Black, PSM, Rear Speaker Kit, Optima...
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