I guess there is a workaround to this problem, that is to say, even if we cannot output 4k @ 60HZ via TB while running Mavericks, we can do this via HDMI. As a previous forumite suggested, all we need is an active DisplayPort to HDMI 2.0 adapter. The important question is whether the DisplayPort side of the adapter needs to be DP 1.1 or 1.2. At the first glance I'd think it must be DP 1.2. But after some thought it is not necessary. Because the already existing DisplayPort 1.1 to HDMI 1.4 active adapter that another forumite pointed out carries the HBR2 feature of DP 1.2. It does not carry the MST feature of DP 1.2, but to my understanding it is not necessary, because the feature that allows for 4K @ 60Hz is HBR2, and not MST, which is necessary only if you want to realize multiple independent video streams.
If my understanding is correct, than NiqueXyZ's concern that an active DisplayPort 1.2 to HDMI 2.0 adapter can be quite expensive is partly, perhaps mostly mitigated. Because all we need would be an active DP 1.1 with HBR2 feature to HDMI 2.0 adapter.
Can you guys (especially NiqueXyZ) comment on this?
PS. Below is the link for the DP 1.1 w/ HBR2 to HDMI 1.4 active adapter.
http://www.accellcables.com/product189.html