Hi , I have missed the post , so my addition might be late or redundant.
I do use a outer ring (and try to avoid using a centering spider if possible to keep the routine as simple as I can).
I must say it works to keep records with slight warp flat. But I think there is more to it then that and I have written a document on the subject, presenting my thoughts and followed up with some experiments and experiences.
I prefer to combine the outer ring with an Oracle and SME style of system; screw on clamp with a specially shaped bottom and a washer under the record.
I regard the ring as the next best next thing to vacuum holding of records, which I have to my availability on 2 of my TT's. I have sent my ring to a work shop for slight modification and when returned (taking some time still) I will also use the ring in conjunction with a vacuum platter to see what benefits may still be present.
As an aside; I am not in favor of dished platters/mats because of the variable azimuth that results from it. A clamp without a dished platter cannot, as skilled its design may be, completely flatten a severely warped record. With a straight, flat platter and mat the outer ring will help considerably in reducing warps.
If you would like to read the document on the subject I could send it. I have translated the doc into English, which is not my home language.