Look at the struct wiced_bt_cfg_buf_pool_t you used for wiced_bt_stack_init call.
As far as I know with 3.5.2 you don't have any hint about their real-time usage.
However you can blindly adjust the buffer config and hope for the best.
I'll recommend you to migrate to latest SDK and get prepared for future migrations.
You will not like to rely on official patches for non-latest SDK...
Kindly update to the latest version of the WICED SDK. The wiced_bt_gatt_disconnect() API should work fine.