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.
You made some mistakes.
If you want to change VM ID you need.
1. Turn off VM.
2. Rename VM config file.
3. Change VM disks location ( in ZFS rename )
4. Change config disk location inside VM config file.
5. Start VM
p.s. why a lot of people do server restart? Do you use windows or what?