I know what you mean. I'm just doubtful it's true because you're the only person I've ever heard that from. None of the info I've read on the Level 10 ever mentioned anything of the sort. I am very interested in knowing where you read or how you acquired the information on which you base your theory? I think there's enough room in the powertube so the carrier is not squeezed, except by the pressure in the dump chamber. I always thought the backing washer was just a precaution against the powertube O-ring slipping into the dump chamber. I don't even have a backing washer, and my L10 still works fine. However I'm not going to consider your theory wrong simply because I've never seen it mentioned elsewhere. I decided to measure all my powertube internals to see if there is or isn't enough room for the carrier to be squeezed or not. Here's what I got:
powertube tip (the part that isn't visible when screwed into the powertube): .311"
backing washer: I don't have a backing washer so I couldn't measure it; estimate: ~.03"
powertube depth (from the top of the powertube without the tip to where the backing washer would sit): .592"
I also measured that the carrier WITH the powertube O-ring in it is actually .218", so the powertube O-ring does protrude about .002".
It looks to me as if there's enough room for the carrier to be free of squeezing. It's close enough, though, that if you put enough shims in there, the carrier probably would hit the bottom of the powertube, and the powertube O-ring would be compress the extra .002" if the pressure in the dump chamber hadn't done so already.
My conclusion is that shims don't affect the inner diameter of the powertube O-ring. They are simply used to adjust at which point in the bolt's travel the leak-hole seals and unseals. Please let me know where I can find evidence to the contrary, this question is interesting enough to be worth investigating.