I agree with much of what you're saying about running Call of Cthulhu, but I fail to see how using the D20 version over the BRP is in any way condusive to the appropriate atmosphere of uncertainty and powlessness. The D&D attitude of the players' primary goal being character advancement achieved through combat, inculcated unequivocally by the D20 system mechanics, which carries over into any D20 system game, is antithetical to this atmosphere. To put it another way, it's not a setting where the players should be preoccupied with 'twinking feats and packing kewl firearms'. I realise that you clearly appreciate this, and wish to make your players do likewise, but using the D20 system, in which combat-oriented feats and unfeasably large quantities of firearms, as a substitute for magic swords, play a fairly central role, does not strike me as the best way to achieve this.
It's difficult to reasonably deny that, even in its adaptation to the Cthulhu setting, the D20 system, which assumes improvement in combat ability as the central priority of character development, and where two-thirds of the feats are combat-related, just providing even more exceptions to an already irritatingly exception-based rules system, is designed for a very different style of game. In next to none of Lovecraft's tales do the protagonists engage in combat, willingly or otherwise, though they often flee from it, or fail to, with predictable results, nor is it, save for a few exceptions, the solution. None of his heroes were martial artists, soldiers or 'hard men' of any sort, rather they were librarians, academics, artists and dilettantes. That these sorts of characters should increase inexorably in combat skill as the learn more of the Mythos, generally through investigation and research, not violence, defies plausibility.
I am not trying to prescribe to anybody how they should play Call of Cthulhu, or with what system they should play it, but, quite frankly, playing it with the D20 system as written will produce, not a game of one-sided struggle against terrifying mad gods in the vein of Lovecraft's works, but one at best reminiscent of Brian Lumley's ill-advised high-fantasy take upon the Cthuylhu Mythos, and at worst, D&D with Deep Ones instead of Orcs, and Shoggoths for dragons. D&D, and by association D20, is a game catering to the 'gamist' stance, Cthulhu is rather one of narrative and situation.
Regardless of your opinion of your players limitations, or lack therof, or any other Keeper's of theirs, for that matter, their refusal to learn any other system than D20, like that of so many others, strikes me as lazy and blinkered, and failing to grasp that different styles of play benefit from different systems is nothing short of idiotic. From what you've said of them, they seem to be stereotypical hack-n'-slash D&D types, if they're not, I apologise to them for any insult, but that's how, by refusing to even consider playing any other systems, especially systems as simple, intuitive and elegant as BRP or UA, they come across. Even if they are, then I've no issue with their enjoyment of that style of roleplaying, it's fine so far as it goes, but it only goes so far, and there's a lot more to roleplaying than D&D, or D20, really make allowance for.
From what you've said, I applaud your efforts to modify their attitude towards the game by emphasising the difference between the premises of the Cthulhu and D&D settings by generating the right Cthuloid atmosphere, and, I very much hope, punctuating your message with the graphic deaths of gung-ho player characters, but you'd have helped your cause from the beginning by using Chaosum's BRP rather than D20. Also, if you prefer for your players not to know the system well enough to metagame, and I suspect that what you mean here, but are too polite to say, is munchkin, then surely employing a system unfamilliar to them would be ideal? If they enjoy it, it might go some way towards showing them that there's much more out there, and that a game needn't come in a heavily-advertised glossy hardcover with full-colour art in order to be worth playing.
To conclude, a roleplaying/musical analogy coined by a friend of mine: D&D is N-Sync (sp?), Call of Cthulhu is Iron Maiden, which would you prefer to listen to?
... and yes there is a roleplaying answer for people who say they prefer Led Zeppelin, but not one for those who say Emerson, Lake and Palmer.