I think that 7500 will be a good deal ... 16 pipelines with 350Mhz + better shaders and less heat ...
Look :
12x450 --> 5.4Gpx/s
16x350 --> 5.6Gpx/s (Good balance performance/heat i think)
16x375 --> 6Gpx/s
16x400 -->...
In fact i didn't try with the battery only ... I didn't read carefully your message.
I will try to boot with only my battery and i'll give you my results.