I don't think either will be panned, if only out of respect, but maybe that is being too optimistic.
No, I don't think Heath's performance will be panned, for two reasons.
One, I think he'll be really good. Everything we've heard from everyone who had any glimpse of Heath in the role says he is amazing. They were saying that even before he died. And you don't usually hear that if it isn't true.
Second, movie critics are usually a fairly intelligent and sensitive lot, and they all loved Heath in BBM, etc. So even if this performance falls below expectations, they would be tactful about it. They might say something like, "Heath's performance isn't as good as some of his previous roles." They wouldn't say, "He sucks." But it's a moot point, because I'm sure he'll be good.
As for the movie itself, it's harder to say. If it isn't as good as everyone is hoping, the critics will say so. But they won't be mean about it, I don't think. All of the people involved in it are well respected; the critics will just say this isn't their best effort, or something of that nature.
On the other hand, I'd be pretty surprised if Heath were to win a posthumous Oscar, or even be nominated.