The reason performance increases with an aft CG is by putting the weight in the back reduces the amount of downforce that the tail needs to actually produce. That part is strictly aerodynamic. So with less down-force needed, there's less drag associated with the tail, and you get maybe 2-4-ish kts improvement. Now the downside is obviously the VMC increase, and also a greater difficulty recovering from stalls/spins.

Elevator feel can also become rather "weak" so over-rotating and flaring could be detrimental if you over-did it and scraped the tail. All this is due to the rearward shift of the CG, and that reduction of the distance between the CG to the tail is now smaller, providing a shorter "arm" for leverage(think teeter-totter with different weights at each end, then shift the balance point closer, too counter-act you'll need more/less weight[down-force] to balance it all out).

I know I over-killed the post, but this is what happens when you're up too late and just looking for something to do. Good luck on finishing out your check-ride.

**Edit: For the leverage, the torque demo above is definately better, but don't forget the aerodynamic advantage of the aft CG**