I posted a question in "General" forum, but i guess it was not the right forum for it.. And no admins have moved the thread (yet).
Not allowed to posts links, so look it up under "Need-for-wifi-speed" in the General forum.
Anyway, i have done some more comparison tests.
Using DD-WRT build 28211 on both the AC68U, and the AC66U connecting WDS i get 200-220Mbps transfer speeds from wired on each side of the WDS link. Thats 10x what i got in Tomatousb using WDS. However, it was by no means stable enough for usage, as the WDS bridge kept dropping and not connecting again until i reboot each router. Connecting as "Ethernet Bridge", i got around the same speeds, but was plagued by the same problems as in Tomatousb - No ipv6 connectivity on the other side of the bridge.
So.. after some trials and errors (had to do a firmware rescue thing from asus) i got the latest Asus (Merlin) firmware up on both routers, and set up the AC68U as "Access point", and the AC66U as "Media Bridge". Transfer speeds are now 450-480Mbps on that link, AND i got ipv6 on all sides of the bridge! :) However, the Asus version of "Media Bridge" does not support connecting wifi clients, so its only lan ports that have connectivity. Ie: Lan ports (no wifi) -> AC66U (media bridge w/ipv6) <-> AC68U (wifi clients w/ipv6).
I will stick with this setup a bit and see how it works, but so far (running some 40 odd hours) i have had no connectivity issues or instability. + ofc double the speeds of DD-WRT, and well 50-60% more than tomatousb.
Both Tomatousb and dd-wrt posts indicate that there is some sort of hardware/driver limitations that prevent the broadcom drivers from achieving ipv6 connectivity across a "bridge", due to it using some sort of MAT (Mac Address Translation). How come asus are able to do this?
One other thing id like to know (but its not for tomatousb dev's to say..) is why the media bridge function of AsusWRT disables my 2.4GHz radio when running in "Media Bridge".. hmmm.