Here are the photos I have for the various headers (Jeal, SWs, Watson, and OBX, respectively). As you can see, the Jeal/Watson/SWs likely have an additional 2-3" worth of pipe in comparison to the OBX design (the Watson photo is of the 1-3/4" primary version, which is quite rare). As mentioned previously, I could see how the shorter tube OBX design wouldn't produce the power numbers of the Jeal/SWs/Watsons.
Also, I ran out to the garage and measured the primary length on my SWs on the drivers side (couldn't get to the passengers) They are:
#1: 25"
#3: 21.5"
#5: 19"
#7: 16.5"
Unless Corey has patterned his 'new' headers after the one of two truly equal length primary header sets ever manufactured (by LPE), his are going to measure roughly the same. I don't own OBX, so I cannot comment with actual measurements, but based on the photographic evidence, they are visibly shorter.
Brian A.
90 ZR-1
12.09@118
ZR WON
01 Z06
not quite stock