2438 Mhz. Which should result in memory bandwidth of 624.1 GB/s.
Spec here:
AMD Navi 32, 2430 MHz, 3840 Cores, 240 TMUs, 96 ROPs, 16384 MB GDDR6, 2438 MHz, 256 bit
www.techpowerup.com
A 2600 Mhz overclock would result in memory bandwidth of 665.6 GB/s.
As far as I can tell, the achievable memory clock depends on the type of memory chips installed on the card, people have been reporting mixed results.
Some cards have memory chips rated for 20 Gbps, presumably some will be 18 Gbps.
RX 7900 XT's are equipped with 20 Gbps memory chips, with a wider memory bus.
EDIT - Looking at the circuit board analysis section for each GRE review on Techpowerup, all the cards they reviewed had 20 Gbps memory chips.
In theory, that should mean they are capable of running at 2500 Mhz.
The only difference I notice, is that the IC chips (for GPU and memory voltage) are rated for 50A on some cards (instead of 70A). Like on the Steel Legend:
https://www.techpowerup.com/review/asrock-radeon-rx-7900-gre-steel-legend/4.html