I wouldn't write off Steeda just yet. Those S550 weight figures could be just for the bare stripped down weights of each trim; i.e. no Premium subtrim/packages and definitely no options whatsoever.
Add Premium subtrim/package and fully option out everything and a GT fastback (not convertible) could hit that +300lbf mark that Steeda is estimating.
With the use of heavy steel to build this car, plus all that extra European pedestrian crash safety equipment, not to mention the heavier IRS, I admit I'm impressed that Ford did not increase the GT coupe (fastback) by only 90-97lbf more than the 2011-14 S197 GT. For the reasons I already said, I assume that the weight given by Jaloponik is for the 301A GT fastback with no options and no Premium subtrim/package.
Still, Ford could have saved more weight. Look what they did for the F150: Going almost all aluminum saved a massive amount of weight from their heavy trucks. I wonder if this avenue could have done the same for the S550? At the very least, saving weight with lighter metal parts could have allowed Ford to put in stronger IRS spindles (halfshafts) so that we Mustang fans would not have to go through the very same suffering that Camaro SS owners have experienced with just bolt-on N/A engine mods.
If I break down to buy the S550, I for one will not buy the MY 2015 S550. I will at the very least wait until MY 2016 when all the weaknesses are known and either Ford makes subsequent improvements or the aftermarket offers better and stronger mod parts than the OEM. My first goal will to get rid of all the weak and underpar bushings in the IRS to get rid of the slop that is sure to be present especially under WOT on even the stock 435BHP Coyote. Ford will absolutely no doubt put in too soft of bushings for the sake of that daily driving ride that they always place above supporting the 400+BHP power of their engines.
My second goal (actually, preferably at the same time as replacing he IRS bushings) would be to get rid of the stock IRS spindles and put in stronger ones to avoid what the Camaso SS owners have discovered.
If Ford executes the S550 production very well with little bugs and if they actually decide to sacrifice some daily driving NVH to give more improved performance and durability, that will be an absolutely pleasant surprise that I will very gladly welcome with open arms; and that just might be enough to push me to buy a MY2015 S550 GT coupe (fastback).