Peter,
6678 supports Jumbo packets, in the past I used it to send UDP packets ~3000 bytes in size using NDK.
The statistics are from the EMAC/switch point of view. For 0x2090b00 region, Rx means Rx from DSP. For the 0x2090C00 region, Rx means from outside network. With the change, you can see the 0x2090b00 and 0x2090b30 increments correctly. Then, you should also see 0x2090C34 and 0x2090c64 increments (Tx to network). If yes, then from the switch, those big packets are sent to network already.
Is your setup verified to be able to capture jumbo packets (assuming you replace 6678 EVM with another device, it sends jumbo packets and Wireshark can caprure it, this is to rule out set-up issue)?
Regards, Eric