Firstly the 2015 iMak 5k that you got is not a Desktop that has PCe3 slots that you can pug the GEFORCE GTX 1080 Ti into. Your 2015 iMak 5k is an “All In One” type computer that has the graphics card soldered onto the motherboard and you can’t just pug the GEFORCE GTX 1080 Ti PCIe3 card into it, firstly, its too big and secondly, you don’t have a PCIe3 on you board to plug this card into your board!
BUT in saying all this you can do it in another way!
You can purchase a “”
Bus Port: PCI-E
Port Quantity: 4 X PCI-E USB
Size: approx. 120 X 66 X 21 mm/4.72 X 2.6 X 0.83"
Net Weight: approx. 41 g
Gross Weight: approx. 73 g
Package Size: approx. 130 X 90 X 25 mm/5.12 X 3.54 X 0.98"
Supported OS: DOS, Linux, Windows XP/7/8/10
1 X 1 to 4 PCI-E Adapter Card
1 X Package Box
You can purchase this from either eBay or Amazon and also buy a case for this card to incase the “Riser Card” and plug the GEFORCE GTX 1080 Ti externally into your “2015 iMak 5k” and use it that way!
This is very simple as I’m using an external PCIe 3 Rise Card on my laptop and I’m using it with USB 3.0 not a Thunderbolt 2.0 or 3.0. It works faultlessly and its giving me resolutions of max 1920 x 1080 @ 60Hz that my Samsung curved monitor can handle (but the 1080Ti has an output of max 7680x4320 @ 60Hz) so remember that. The card that I’m using is a MSI GeForce GTX 1050 Ti 4GT OC (Base: 1341MHz,Boost: 1455MHz), 4GB GDDR5 (7108MHz), PCI-E 3.0, DVI, HDMI, DisplayPort and it works great with much superior graphics that my laptop has. Hope this helps and cheers