Having seen the film (and having been very sceptical to it before I saw it, as you may recall), I'd say the film would certainly have garnered very wide attention and adulation no matter what. For a superhero movie it's definitely of decent quality. Not my particular cup of tea, what with the character development largely getting lost in endless car chases and explosions, and most of the characters not rising above their 2-D cartoonish origins. But there is no doubt the film is good enough, on its own premises and in its genre, to have merited positive reviews and a big box office no matter what. That said, the added poignancy of this being Heath's last finished film of course hasn't decreased the interest. But in my opinion, it has been one factor among many. At the end of the day, the film stands on its own two legs.
As for Heath's performance in it, it is nothing less than astounding. I disagree that it is his best performance to date - that would still be Ennis, no mistake about that - but the Joker is an immense acting tour de force, that is indisputable. There really isn't much grounds on which to draw a comparison between Ennis and the Joker, they're that different and at opposite ends of the acting range. The main difference to me is that I can (and always will) relate to Ennis as a real person. With the Joker, for all his impact, I couldn't do that - he was a larger than life, utterly impressively acted cartoon figure. Not a real person, never a real person, but a riveting representation and symbol of everything we as individuals and as a society fear that goes bump in the night, everything that challenges our humanity. A symbol of the forces of chaos and anarchy, and their mysterious attraction and lure. He has such incredibly sinister presence and power and sheer frightening charisma in the role, he nearly burns through the movie canvas just from being present on screen, never mind the incessant explosions in the background.
Whether the performance is Oscar-worthy.... well, I happen to have no respect fro the Oscars after 2006, so I don't know. That term is not my term for a good perfomance anymore. And the Joker is a unique performance. I don't think Heath's tragic and untimely death has changed my view of that one iota.