[CentOS] Jumbo frames problem with Realtek NICs?

I was trying to do some performance testing between using iSCSI on the
host as a diskfile to a guest vs the VM guest using the iSCSI device
directly.

However, in the process of trying to establish a baseline performance
figure, I started increasing the MTU settings on the PCI-express NICs
with RTL8168B chips.

First bottleneck was discovering the max MTU allowed on these is 7K
instead of 9K but googling seems to indicate that the RTL8168B is only
capable of 4K frames.

I assumed 4K would still be better than nothing but unfortunately
bumping up the MTU to anything else but 1.5K caused the file transfers
(using NFS for easy testing), to hang at random points or more
accurate slow to a crawl.

Checking the syslog, I discovered warnings that increasing MTU with
this adapter may cause problems.

Searching around, it seems to be a common problem but there doesn't
appear to be any clear cut fix, including some suggestions to use a
third party driver.

Does anybody know of a proven solution or is the Realtek chip itself
irrevocably broken/bugged that anything above the default 1500 will
simply not work?