[susan53]

This sort of backshift, which makes the verb in the reported statement conform to the verb in the reporting statement, is optional when the proposition in the reported statement is the same at the time of reporting as it was at the time of speaking. That sounds complicated, so a simple example to start :

If someone says on Monday I'm going to London on Thursday and I report it on Tuesday I can say either He said he's going to London on Thursday or He said he was going to London on Thursday - because the proposition (here that the event is a future arrangement) is identical. If on the other hand I report it on Saturday, then the proposition no longer refers to a future arrangement but to a past arrangement, so He said he was going to London on Thursday becomes obligatory.

Now your example : He will be performing here. This time we've got a prediction of a future arrangement. So the same thing holds : if you report the statement when it's still a prediction, then you'd have the choice between I heard he will .. and I heard he would .... Once the prediction and the event have become past, then would becomes obligatory :
A : I went to the Eminem concert at the stadium last night B : Oh yes, I heard he would be performing here.

So far, we're just dealing with facts - but what if you want to throw doubt on the truth of the proposition, as you suggest? The second form of the verb is not only used to express past events (as in the examples above) but also hypothetical events : If I was rich .... (= in reality I'm not); I wish he would shut up! (= in reality he won't).

So, when I have a choice between the first and second forms (in my example is/was and in yours will/would) for the reasons expressed above, by choosing the form which can also express hypotheticality, I can throw doubt on the factual truth of the proposition. You'd probably combine the choice of verb with an intonation pattern that reinforced this sense of doubt though (drawl on heard ???), and maybe even an explicit statement afterwards.

A : It says in the local paper that Eminem's doing a concert here! B : Yes, I heard he would be performing here too - and believed it till I realised it was April 1st.

So - the second form doesn't necessarily express doubt - it may express the fact that the event is now past, or it may simply be an unmarked optional choice. But it can be used like that if the speaker wants to.

Hope that helps