The L5 uses metal shields over the chips and thermal paste to dissipate heat through the aluminum frame and the metal back of the LCD, which is why the sides and screen of the L5 gets hot. (One of the benefits of adding a case is that your hand doesn’t have to touch the metal sides of the phone when it gets hot.) I doubt that just drilling holes in the case will do much to effect the temperature, but I look forward to hearing about any tests that you do.
If you added a tiny fan to suck out air and drilled holes in the case so air is pulled from the outside and over the surface of the PCB, I think that would cool the L5, but it would also shorten the battery life.
In my tests, the L5’s processor was limited to a max temperature of 50C, which is 10C cooler than the max temperature of the PinePhone, but you feel the heat a lot more in the L5 because your hand touches the metal sides and LCD, whereas you can avoid touching the upper back section where the PInePhone gets hot.