Well, the washers might not back out but what about the elastic / plastic characteristics of the studs / cap bolts? What is the prestress on your cap bolts? At what torque? Are you beyond the elastic limit? How are you allowing for the 3mm growth of the manifold and staying within the plastic limit of the fastener? and still fit within the top #3 #4 physical limits?
The locking Drake Nuts are only a small fraction of the fastener system. Maintaining the torque (holding force) of the manifold is the important part. Why do you think DC put such a long bolt and such a heavy sleeve as OEM? Because it can stay elastic with 3mm growth and maintain torque because of its length and sleeve.
The problem with the ATS is the top #3 #4 not allowing a long bolt and sleeve. The shorter the bolt the less total growth length it can stand and stay elastic. That is why the Belleville washers, that is what they do, that is what they are designed to do. The top #3 #4 limit the length of the holding bolt to about 52mm. With a 52mm length, a bolt can not stretch and shrink 3mm and stay elastic (ie hold torque when cool as well as hot). The lock nut is whatever you want to use. The Drake is a good lock nut, but not the only one for sure. However, the Belleville washers is what allows the 3mm growth and shrink and still maintain torque.
The stainless steel washers just don't let the Belleville washers cut into your manilfold boss. Of course if you do not care about the Belleville washers cutting into the ATS manifold boss, do not use them.
I would really like to know what system you have to attach the ATS manifold, maintain torque, NOT go into plastic conditions on the fastener, and fit within the confines of top #3 #4.
I did not go this route because it was easy. It turned out to be significantly more involved than I would have ever thought. However I tried every thing I could think of before I went the Belleville route.
Good luck,
Bob Weis