overfueling of a stock IP will cause heat mainly because the additional fuel means longer duration of injection. End of injection is getting late, causing black smoke and heat besides some extra power, plus the plunger speed is beyond its max. (cam profile/plunger travel). That means the additional fuel is injected late and slow. All that leads to more heat compared to the power gain.
With correctly calculated elements end of injection is just before/at the max, speed of the plunger. More fuel is injected within a shorter time and can burn better due to better atomisation and more "distance" to TDC (more time).
Now given the same amount of air with Deni's stock engine he can burn a little more fuel with larger elements due to the above said.

Tom