I think the only way to optimize this space further is to remove the spine..
Think about it this way. Sure if i remove the "spine" (although its more of just a motherboard tray) i might be able to position the gpu and motherboard closer together.... And this will allow me to probably squeeze in a 3slot design correct? But for example in the T1 in 3slot mode, cpu clearance goes down to 50mm, which starts to limit your options for pump/block clearances. And then there's another factor, the case length... Almost all 3slot cards are above 300mm in length, so even if i do remove it and accomplish a 3slot design, there will be very little use of it... And as I've stated before, 2.5slot is enough clearance for fittings if you want to watercool and the new iteration im on has alot more room for GPU height which also helps for water blocks.
And then of course there's structural integrity... You're over looking the fact in cases like the T1 and the winterone that use "struts" have a thick front and back panel (i think this ranges from 6-15mm) along with thick struts that can support alot of the weight.
If i do the same with 1mm or 1.5mm sheet metal, thats going to over time start to slowly bend with the weight.. theres a reason why i got weird bends in the different places... Each bend adds to the strength of the sheet metal... So it might not look like it, but have a piece standing right in the middle of the case, that's bolted into the bottom, back and the radiator support, adds quite a bit of strength.