Your baud rate limit might be determined by the maximum time spent with interrupts disabled, along with your ISR execution times (including saving/restoring registers used by the ISR).
I can't help with providing hard data on how much time you'll have to service serial ISRs while running Ethernet and/or Wi-Fi code. Up to a point, you'll just see slower data rates on the network connections.
You might just have to write a torture test that floods the serial ports at your maximum baud rate while trying to maintain a TCP connection over both Wi-Fi and Ethernet. Use WPA2 Enterprise for the Wi-Fi to incur additional overhead during the testing.