Hello Xinke,
I'm a graduate student at the University of Washington, and just brought up an EGPRS/GPRS network with an Ettus B210/osmo-trx. My B210 has a GPSDO as well. I'm just getting started on this project (so I'm unfortunately not the expert you are looking for), but I can corroborate that I am also seeing very limited single user throughput in EGRPS and vanilla GRPS (6-8kbps maximum measured via iperf). My network is built with the latest split components running on a debian9 virtual machine via virtualbox. I'll attach my configs here if they're helpful. I also have access to a litecell1.5 and will be bringing up the network on it in the next few weeks, so I'll be able to report if it behaves any better. The litecell setup will also be running on dedicated hardware to rule out any strangeness from virtualbox. Sorry I don't have any other clues at this time, but I can at least confirm Neels's analysis above and rule out the impact of running the old nitb and ggsn.
Cheers,
-Matt Johnson