Yellow Sign said:
But what Keeper worth his/her salt would run a d20 CoC game with 15th level characters?
Because they're there.
I feel that (1st to 3rd level characters are) more representive of a normal person.
True, but I was pointing to the difference between a BRP Cthulhu character with the equivalent combat experience of a 15th or so level character. The d20 version will exude a different feel and be somewhat tougher than the BRP version, who has no feats, specials, or hit points to fall back on for cuts and scrapes. A high-level d20 version (remember, because I'm talking about equivalence in combat-readiness) will take more scrapes than a BRP version with, say, 80% to hit with a rifle or pistol and a 70% dodge skill. The BRP character, no matter what, will only take one or two pistol hits at most before passing on; the d20 version will take more than that at high level, because the firearms and weapons do equivalent damage, but the d20 version has many more hit points and a failure chance of 1 in 4 or less to fail a damage save for any hit of 10 hit points or over. He also has feats like drive-by shooting, cleave, toughness, etc. to fall back on.
A BRP version character will likely go down on ONE 10-point hit, even if he has hundreds of percentage points in skills (the quivalent to a high level d20 investigator).
I'm not disputing the lethality of the d20 version at all - I've played both. But I've also seen characters be more reckless with their low-level d20 character because they FEEL better about the character in question. It's kind of like watching a person go into a dangerous situation with NO body armor, versus a person who goes into the same situation with a bullet proof vest on. BOTH still stand a good chance of being killed, but the guy with the vest is slightly more protected, and at least feels better about it.
Also, Cthulhu could crush a 15th level character just as easily as a 1st level one.
Despite what some d20 haters out there may say, this is WITHOUT question.

I'm referring more to facing crazed cultists, deep ones, and ghouls than things like tindalos-hounds and shoggoths. No matter WHAT version, if you see a shoggoth, and don't lose your marbles, getting the heck out of dodge is still the best plan.