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The Ford 9" rear is one of the least efficient of the common mass-market rear axles. It is a semi-hypoid, where the pinion gear is further offset below the axle centerline than a pinion would be in a typical helical bevel gearset. Hypoid gears are at least 3% less efficient. The Ford 9" rear is popular among hot rodders because it is common (i.e. inexpensive and its ubiquity provides many gear ratios to choose from), it is a "quick-change" style rear (the ring/pinion/carrier is assembled to the pinion housing as a unit which drops into the axle housing), and the low pinion allows driveshafts to clear in low-slung street rods and lowriders. Efficiency has nothing to do with its popularity.
A GM 10 bolt or even a 12 bolt would let you recover that 3% efficiency loss from the gearset alone. It would be a pretty big deal when you're talking about gearsets that are about 94 vs. 97% efficient.
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