Your RDA top cap is just a hunk of (likely) metal which is going to conduct heat; RDAs themselves will get fairly hot during normal usage and some of this heat will transfer to the mod, as well. I don't want to suggest that we shouldn't be paying attention to how hot our toppers and devices are getting or that you shouldn't be concerned if your device itself seems overly hot, but "my RDA gets hot while vaping" is just a fact of using RDAs and not necessarily indicative of a problem.
That said, harsh hits are not good and a definite sign that something isn't working right, though it may be as simple as using more power than your coil can handle.
Besides what sort of wire type and gauge you are using, what is the resistance? 40W pushed through a 1.8ohm coil made with 30g wire is VERY different than 40W pumping through a .2 ohm made with 22g.
Hot legs can also cause harsh hits at any power level.