It's good to have a balanced platform.
It doesn't make sense to use a more powerful motor than your batteries can support.
Generally the larger the battery (of a particular chemistry), the more power you can get out of the battery. If you're going for more power, you might opt to use a power chemistry (which offers more power/weight but less energy/weight = range/weight). So there's a power / energy / weight tradeoff.
The heavier the battery and motor you use, the heavier you need to make the frame.. and then you need heavier brakes, heavier shocks, etc .. and all the weight further reduces low speed range. We've seen this with the Zero bikes - they've gone from the 2009 S @ 225 lbs, 3.6 kWh to the 2012 S ZF9 @ 341 lbs, 7.9 kWh.
Despite the weight gains, the Zero bikes have supposedly gotten more efficient over time.
The 2009 S was
rated at a highly optimistic "up to 50 miles" (72 Wh/mile).
The 2010 S increased to 270 lbs (though I wonder if the 2009s actually weighed 225), no change in battery or maximum range.
The 2011 S increased pack size to 3.9 kWh and weight to 297 lbs. EPA UDDS range is 43 miles (91 Wh/mile), maximum range is highly optimistic "up to 58 miles" (67 Wh/mile).
The 2012 S ZF6 increased pack size up to 5.3 kWh, weigh still officially at 297 lbs. EPA UDDS range is 76 miles (70 Wh/mile).
There's no all-round "best" combination of attributes .. but for a particular goal (say racing in the IOM TT) there may be one or two locally optimal configurations. Generally speaking the race bikes have gravitated towards large pack direct drive high RPM AC motors. One exception is Kawasaki's entry, which uses a ZX-10R frame, fairings, and transmission paired with a slightly smaller battery pack.
