EZ-Serial does not currently support the Data Length Extension of Bluetooth 4.2. This is partly due to RAM availability (I believe it may not be possible to implement on 128k modules with only 16 kB of RAM), and partly due to feature prioritization. At the moment, we do not have immediate plans to add support for it.
Because of the API-based mechanism required to use L2CAP, there is some extra UART packet and processing overhead involved compared to the transparent CYSPP mode, and it will be difficult to beat that throughput. Avoiding errors to reduce this overhead--as you are already doing with delays--will be the best approach. The errors come directly from the BLE stack that EZ-Serial uses, so there's no practical way to avoid them otherwise.