First, is your connection using PPPoE? From the MTU value of 1492, it looks like that. If it’s PPPoE, you can try to increase the MTU of the pppoe-out interface to 1500. I’ve a previous post about this topic in another thread:
http://forum.mikrotik.com/t/trying-to-understand-the-need-for-mss-clamping/175749/1
2nd, do you have Fasttrack enabled on the hEX S. If it’s not enabled, then the measured speed might be normal for the hEX. That’s about near the limit for hEX/hEX S without fasttrack. Remember that now the router also has to perform PPPoE encapsulation/decapsulation, which uses a non-negligible amount of CPU power. So if previously it was near 100% limit at 300Mbps without having to handle PPPoE, a throughput of 280Mbps with PPPoE processing looks understandable.
But if you have fasttrack enabled and active, then the CPU won’t be at the limit with those speed. It could be that previously the Huawei modem could use MTU 1500 with PPPoE and now your router doesn’t. Try to increase the MTU like I wrote in the other post first.