I performed some quick experiments of search time vs channels found. Here's the results:

Search Bands | Channels found | Time
-------------+----------------+--------
    21       |      6         |  48.202
    31       |      8         |  65.194
    41       |      9         |  92.639
    51       |      9         | 103.231
    71       |      7         |  99.600

It looks like 31 or 41 is a good balance between runtime and performance.