here i describe my fun with MTU and the stock webinterface of the LS Pro:
http://forum.nas-central.org/viewtopic. ... &sk=t&sd=a
somehow i think that the arm9-boxes only allow specific MTUs maybe?
try the MTUs that are mentioned in my first posting...they work on the LS Pro and can be set via the webinterface of the stock firmware.
look at http://www.debianhelp.co.uk/mtu.htm
/sbin/ifconfig eth0 mtu <custom-MTU>
that only does it temporarily...if it works at all. in case you just loose network connectivity and you cannot login via ssh anymore it should be possible just to power cycle the box to get it back in its working state.