The Socs mainline implementation and drivers have not been done yet and that is going to be a long term thing whilst it runs a Rockchip BSP image (Board support package)
So it will run at a basic level for some time to come but likely someone somewhere when things get more stable will tinker with the OPP table.
2.8Hz is published so likely and often its very easy to get 10% past official max, so thinking 3.0Ghz is very likely if you can keep it cool.
But because it is on a BSP and likely to be in dev for a considerable time I doubt they will change from the default safe frequency of 2.4Ghz until board has been released and stable for some time if they ever do.
3.0 Ghz is a guestimate but for a desktop/sbc board where you can get the cooling I would have a bet its possible and if in 6 month time the board is released I bet pretty quickly someone is tinkering with the OPP table to somewhere in that effect.
Often there is a community DTB to enable faster clocks a bit similar to Raspberry and their config.txt OC, Rockchip state 2.8Ghz so prob take that for definate and as said 10% is very likely.
Doubt they will change from 2.4Ghz at this early stage though as they play safe and see how things go.
All the boards Radxa do, as far as I know have had a community overclock and generally Radxa pick a fairly safe default.
Last one I did was the Radxa Zero which runs stable @ 2.0Ghz
Its been so long can not remember what the initial clock of the rk3399 was but it did get bumped to 2.0Ghz so maybe it was 1.8Ghz initially.
The OPP table that sets clock is ‘software’ the Device Tree Blob DTB is always easily hacked.
It might be my memory as thought I saw 2.6/2.8 in the datasheet but can not find it maybe it is 2.4/2.6
2.6 + 10% = 2.86 so maybe that might be max we will get, maybe the base 2.4 they have chosen is max but whatever it is it will just be a matter of software.
I was having a look to where I saw 2.6/2.8 and maybe it was a figment of my imagination as can not find anything