Does using a case with iPhone cause excess heat to linger around and does it have any significant impact on battery life?
Whether or not the case can cause a thermal problem depends entirely on the case.

Is it possible for a case to impact battery life by not dissipating heat - yes.

Is that true for any case - no.

Imagine a well fitting case made of a metal - it would easily dissipate just as much heat from the phone as when not using a case.