So I can tell you how I go about making that decision but not sure anyone else does this.  I am just going off the numbers on my mechs but DL8 does about 14k damage in a magazine while DL12 does 20.5k and RPG4 does 5.5k.

An easy way to find out damage to energy ratio is just do some basic math and divided by the energy capacity to determine how effeciently the weapon(s) uses the spot.

DL8  (Rank 6 Level 4) & DL8  (Rank 6 Level 4) 

(14k +14k) / 16 = 1.75k d/ec

DL12 (Rank 6 Level 1) & RPG4 (Rank 2 Level 6) 

(20.5k + 5.5k) / 16 = 1.63k d/ec

So based on their current level two DL8s will out perform a DL12 and an RPG4. 

On a higher energey capacity mech where you are not using the full energy capacity with DL8s they become less efficient and DL12's are the better choice.

DL8  (Rank 6 Level 4) & DL8  (Rank 6 Level 4) 

(14k +14k) / 24 = 1.16k d/ec

DL12 (Rank 6 Level 1) & DL12 (Rank 6 Level 1) 

(20.5k + 20.5k) / 24 = 1.71k d/ec 

Hope this helps!