Yes, more disk = more speed/IO. But creating ZFS you must think about how much time you have until you change death/damaged disk. Like Raidz3 give more time compare to others options. I had strange situation then one disk go offline because of unknown reason and ZFS stops writing to it. Pool...
You can change ARC size live with:
#echo 2147483648 > /sys/module/zfs/parameters/zfs_arc_max
#echo 2147483648 > /sys/module/zfs/parameters/zfs_arc_min
if you want fixed ARC size. I use /etc/rc.local for custom tuning the system.at startup.