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hpel and qpel _are_ sub pixel resolution.
The benefit from it is the more precise movement detection - thus possibly saving texture bits, which get consumed by bigger motion vectors usually.
The error that this more precise detection produces is smaller, that's why it's considered to improve something(the amount of error correction data gets less). You can see the success of it in the sharper image you get, it has more details - even if you have to compress the material more.
The search range is still the same:
if in fullpel 32 pixel range is searched, this happens in hpel and qpel as well (hm. not sure about the hpel part). So the search efforts increase (think of it as if you'd blow up the image by the factor of 4 in each direction).
I hope i didn't cause more confusion now %)
Best regards
Koepi
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