I'm currently using .25mm brass shim that's 30mm tall and see no change in spikes from center to edge of frame if i move the same star around and that's with a 10" f4 mirror.
I have thought about going thinner but i think the secondary would need a counter weight out the front.
http://farm6.staticflickr.com/5527/1...85c601c7_c.jpg
IMG_9149 by
Raymond Collecutt, on Flickr