Hello,
Are they 2Gb cards ? It sounds that's some RX550 or 560 with those speed.
With 2Gb i've a stable hashrate with multi-hash (intensity) of 432, and 448 or 464 when no screen is plugged.
You've a rig of 13 cards, that's big under windows, maybe you reach some drivers limits somewhere. JCE is a pure OpenCL miner, i let OpenCL and the drivers allocate resources, so if your driver makes your hashrate drop, JCE tries nothing to prevent it. Safety first.
I'm building 0.31f right now, with:
Linked against official AMD SDK 3.0
Invalid shares detected by CPU now logs the Thread and Lane (good for memory diagnostic)
Still new OpenCL code, it's good on all my cards (HD7000, RX550, 560) but blind-experimental on RX570, 580 and vega. Reads: perfs may be worse than 0.31c
edit:
0.31f online