yes, #3 confirmed.  I recall there was something analogous with light probes or IBL, where you (bill) and/or mec3d (catarina) showed that a lo-res image would suffice.  in this one I multiplied the jpeg by 2.2 to get it to approx. the same brightness as the hdri (dosch-extreme-hi-res), which was 6250X3125 and in photoshop showed a range from (0,0,0) to <(20,20,20).  it seems many of these hdri don't have a good range.