Grinding discs will "load up" with the aluminum as it gets hot. Same with a regular double cut burr bit meant for ferrous/hard metals. The other downside is if you need to weld the aluminum afterwards, grinding discs leave a lot to clean up. Ideal cut for aluminum would be a band-saw, a single-flute burr bit meant for soft metals, or a regular file meant for soft metals as well.

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{2010 Mazda 3 266,000mi} - daily since new, Airlift suspension + some chunky sway-bars: fastest pass 17.7@77MPH
{1987 300SDL 256,000mi} - stock turbo @ 22lbs straight piped + a whole bunch of pre-turbo water/meth injection, 722.3 with increased line pressure, special sauce, and shift kit: fastest pass 16.5@82
{1992 964 Carrera 4 Frankenstein ??,???mi} - 993 forward dated, Speedster conversion, GT45 + E85: fastest pass 12.5@113