I believe syslog and console buffer are both stored in RAM. So, the only issue with increasing their size would be to limit the RAM available to other system needs. Which, if the network device doesn't have a need for that RAM, shouldn't create any problem.
What I don't know is whether RAM is immediately set aside when configured for a specific size or whether it's allocated as needed. (I suspect the latter.)
BTW, console output could cause high CPU usage. I.e.:"
Possible Causes of High CPU Utilization Due to Interrupts
High CPU utilization on an interrupt level is primarily caused by packets handled on interrupt level. Interrupts are generated any time a character is output from the console or auxiliary ports of a router.
" Don't know if this still holds, but if it does, it would be best to avoid doing console logging.