Siera had no useful information on their site about their pads so I sent them an email.
Did you check that the pistons them selves are square to the rotor face?
If all this doesn't work, you just have noisy pads!
I am assuming you have Wilwood calipers correct?
Here is some info from EBC on the subject:
http://www.ebcbrakesuk.com/catalogue...4_jan_2001.pdf
The rear calipers, those are the standard two piston girlings that have been split and bridged for the thicker rotors? If so I think the EBC part #s will be DP2101/3101/4101 (green, red, yellow) 80x66x15mm pad.
Rick