hi there!
cheapest way to get more power for mac, would be firstly topping out Your machine (pair powerful GPUs + external PSU & it could be small one, like SFXL size unit). if that's not enough, then expanding is an option as well.
However You should know couple of things first. You get what You pay. By that I mean, most of expansion boxes will set You back for at least like 1-2k$ & the reason being is that those have 8 or even 16X PCIe upstream interface - actually worth the price, but well.. You might be looking for something cheaper & if that's the case, look for something like
splitters just keep in mind that You would need to figure out some sort of mounting solution.
GPU cluster helps to solve that, sort of..but it's still a bit messy for some, so Guys keep putting this into some box.
alternatively, You can grab
backplane. This actually have holes drilled that are compatible with (m)ATX cases, so You can mount this into any regular case, add power supply & You would be good to go. You can work up to two cards on air, or up to 4 if You single slot mod & watercool them. Bellow You can see some shots based on this backbone in a case with custom watercooling.
it works great, on air, with gap in between You can get cards cooled just right. Macs seems not to control GPU fan speed properly so from something like TitanXs You can expect like 130 on first run & down to 111 after a while (980Ti would be a bit slower).
As for slower interface. if You push higher resolution to render GPUs might start to have some gaps in utilization & You would notice some speed drops, but that comes to the point - You get what You pay. Another thing is that some functionality like baking might cause system shutdowns (kernel panics) so if You really care about stability, better build a proper render node.
I haven't looked to this lately but seems that macs work best on Maxwell cards. Those are getting harder to find new, but if You would look around You can get pair of second hand TitanXs for instance - just few days ago guy on our community was selling pair of those for 900EU (in total, with shipping). So if You would get a good deal, You can grab a pair of good cards + PSU + backplate & case for like way less than 2k$ & that would lead between 220 - 260 in terms of OctaneBench, 12 GB of vram. If You want to cut some corners, then 980Ti is a great option as well. Try to do some research if Pascal based cards are now supported..maybe You do not need to stick with Maxwells anymore, however for the price some cards are sold now it really hard to resist..I almost bought those two TitanX I mentioned..but then realized 1080Ti is already coming & then there will be other expenses..so passed that.
Hope that helps ;) Let me know if You would have any more questions regarding to this =)
