which order for encryption?

[soomon]

I hope this is the right section, as i am more asking from the performance point of view.

i want to encrypt my data partition.

i was thinking:
disk > software raid 5 > encryption > lvm > ext4

now i read that performance is much better when you do:
disk > encryption > software raid 5 > lvm > ext4

because the system spawns 4 threads for en/decryption.
my source is: http://superuser.com/questions/305716/bad-performance-with-linux-software-raid5-and-luks-encryption

my system has a Intel(R) Core(TM) i5-2400 CPU @ 3.10GHz
so it has AES-NI.

is that true?
if i choose the same passphrase for all disks, do i have to enter the password multiple times or is once enough for all disks?

thanks & greets,
soomon