"I would think the Rainbow design would have the least sag just due to its pole design"
Never actually measured that but it isn't my impression having used both the the Contrail and the Rainbow.
What I think is happening is that with the Rainbow the sag is less obvious because it is divided into two halves, in fact with the cross strut , four quarters.
BTW, last night I was watching a docu on the race to the South Pole called "On thin ice" (http://www.telegraph.co.uk/sport/othersports/south-pole-race/), well there was plenty of sagging there (tunnel tents)
Franco