Anyway, once again, can we please lay off the edition comparisons here? This should not be an edition specific thread.
Sorry for any part in any derail; just an interesting aside.
The X/Y factor doesn't need to be edition specific, but it is a function of rules (as is lethality) that affects roleplay.
I am not sure that the level of lethality directly affects roleplay, except in extreme examples. If jumping into combat is pretty much death, you will either avoid combat (if you can) or not invest in the character (if you cannot). A horror game or a funny game (like Paranoia) might work well with this level of lethality, but it is not conducive IMHO with S&S gaming.
I have no problem with character death, as you well know from previous discussions, but I do think there should be some cushion between life and death in the game. (Hence "Shake it Off" in RCFG.)
Of course, the other extreme is equally problematic IMHO. If combat isn't lethal enough, there is little reason to do anything but solve problems with a sword.
Now, I am not one of those who divides the game between combat and roleplaying, and I like a ruleset where the combat rules are simple enough to not break immersion, so that the demarcation between combat and non-combat isn't so sharp that fighting feels like another game. I believe that you can have a lot of roleplaying during a lot of combat, so long as (1) the combats don't grind on and on, and (2) the rules are light enough so as to not break immersion (a very subjective point).
I also know that there are some games where there is no real lethality, but that still work. As with ultra-lethal games, there are exceptions to the general rule. In a Doctor Who RPG, for example, PC death should be extremely rare (within the show's history: only Katarina, Sara Kingdom, Adric, K-9 Mark III......you might count Captain Jack Harkness, but as he keeps coming back to life, I do not). (A Torchwood RPG would be more lethal.)
Sorry about the rambling.
In conclusion, though, within a wide range of possibilities, I do not think that the level of lethality necessarily affects role-playing. It may do so at the extremes. IMHO, there are other factors than lethality related to combat rules that affect level of role-playing far more.
RC