Is it possible to change the MTU values of the RF850?

Yes, you will need to do this maually via ssh.

To make this change permanent, the following line should be added/modified in the file : /etc/sysconfig/network-scripts/ifcfg-eth0

MTU=1492

This will take effect after reboot.
Run ifconfig eth0, the MTU shown will be 1492.
Change eth0 to eth1 or eth2 as necessary.