Proxmox 4.0 with zfs raid 1

I'm testing Proxmox 4.0 with 2x 1TB sata disks and ZFS raid 1. In the past with Proxmox 3.x and mdadm softraid the overal performance was better. Zfs takes a amount of cpu time. Can the performance with zfs improved by simply adding a SSD disk as cache?

With zfs sync=standard (enabled)

FSYNCS-SECOND: 105.66

sync disabled
FSYNCS-SECOND: 16099.22