When comparing your Setup with others from the MadOnion's Database, you have to make sure their Systems used are running close to, or (ideal case) at your own Specs.

Check their CPU/System used and make sure they state that they did not Overclock anything.

(you want to avoid comparing your System, which seems slow with 26fps in Car/Low Detail, to a High-End System possibly running a lightning fast Pentium4 or AthlonXP)

The flashed Mac Voodoo5 should perform similar or identical to a PC Voodoo5 (assuming identical Bios used), unless it runs on lower Clockspeed (not sure right now, I assume the Mac V5 also runs at 166MHz).
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As far as I know, the Mac Voodoo5 should have the DVI Connector the PC Variant doesn't have. That should be (technically) the only difference.