>It isn't possible for me, as an English speaker, to process those as the same sound (although prolonged exposure to Japanese might eventually do it; I've become much more comfortable with losing the s/sh distinction as a side effect of exposure to chinese speakers

I can confirm that it happens: if I've been listening to Japanese for a few hours I find myself struggling to distinguish the first few r/ls I hear in English. Even more bizarrely, I've noticed my speech drifting, vocalising an intermediate sound when I'm saying an English world with an l or r in. Strange how the mind works.