Low DL Throughput - LTE 4x4 MIMO 64QAM

Hello Experts.

For low DL Throughput (~225 Mbps, expected 291 Mbps) using 64 QAM configuration with 20 MHz BW 4x4 MIMO on LTE eNB I see very bad PUSCH SINR (-18dB avg) and considerable BLER (8% avg with many spikes during the interval) but other parameters in expected range (CQI=15, numRB=100, RI=4, numCW=2).

Can I suspect the SINR to have caused this low DL Throughput?
If not, what other parameters can be checked for here?
Please note this test in ideal conditions in lab with single UE and only one cell up.

Any pointers would be helpful.