I bet you won't notice a difference between that and a Kemper straight to PA except for the feel in the room. It wouldn't be fair to compare PA + amp in the room to PA only. It'll never feel like the amount of air that a guitar amplifier moves through a real cab........
.......In a live setting, running a Kemper through the power amp of an amplifier sounds identical to the amplifier itself. So the difference comes mostly from the power amp. Surprise! A class D power amp doesn't sound like 100 Watts of tubes... But even then, with some minor tweaks you can get a Kemper to sound reaaaaly close.
As mybyte says, the sound and feel of the KPA through a guitar cab is effectively indistinguishable from the real thing. I have various Mesa Boogies that I like and a pair of Mesa EVM12L Thiele Cabs. I have done comparisons where I made direct profiles of the amp and then used an A/B box to switch between the Direct profile in the KPA Powerhead driving one cabinet and the real Mesa driving the other. If anything, the KPA actually sounded very slightly better but that was so subtle it might just have been position of the cabinet in the room or the refining process. Basically on a live gig situation I wouldn't be able to tell the difference.
I've also done the comparisons of my own miced cabs and a Studio profile of the same set up. As mybyte says, they are effectively indistinguishable. Clearly the sound of the full signal chain that the audience get through FOH and the direct profile with traditional guitar cab are different beast but if you want the feel and sound of a traditional guitar rig on stage with the benefit of sending a full signal chain to FOH making your own profiles will definitely get you there.