Giant is a great company with a lot of cool innovation, but I'm not a fan of Overdrive. BTW, those quoted dimensions look wrong to me - it's non standard, and those numbers are pretty standard. I thought all Overdrive forks were 5/4" upper.
Stiffness to weight of a straight 9/8" steerer is pretty good, and with a 5/4" or 3/2" lower it's plenty. Include aero in the consideration, and it starts looking a bit silly; IMO it's just an excuse to lock folks into using proprietary forks and stems.