aside of what rando289 said about not reaching the theoretical maximum, you're mixing up units. You'd have to divide by (1000^3) as bits are always measured in proper SI units (1KBit = 1000 Bit).

TB does 20 GBit/s

And 5120x2880x24x60 Bits/s / (1000^3) is 21.23 GBits/s, so even in the best-case scenario (which is likely unrealistic), you wouldn't have enough bandwith over TB2.

DisplayPort 1.3 will solve the issue of course, but that doesn't help me personally much as the hardware I'm using is a 2012 retina MacBook (which I'd be theoretically willing to replace to be able to use the resolution we're talking about here) and a 2013 MacPro (which I won't replace this early), neither of which have DP 1.3 support.

I'd bet against it. Those kind of specs are almost never reliably achieved. Besides, "DisplayPort version 1.3, which was expected to be finalized in Q2 2014, will increase overall transmission bandwidth to 32.4 Gbit/s" wikipedia