I think 4 Hispanos mk V are better than 8 m2 because
1 hispanos have more penetration
2 high explosive hispano shells are much more effective at incapacitating personel . Acording to an article in wikipedia , a high explosive round from the m 61 Vulcan ( though not a hispano mk V , its a 20 mm cannon ) has a 2 meter effective lethal range ( although this doesnt mean the hispano does the same damage , but it allows us to make a approximation of the damage)
a 50 cal , on the other side just penetrates without explosive damage .
So , in a anti convoy mission , I think the hispanos , firing less rounds but possesing higher damage potential will be more effective
Also , I dont think that we should calculate the total kinetic energy , and momentum of theese shells/rounds because many of them will just go through the vehicle , without expanding their entire kinetic Energy and momentum . The proper method to follow is to calculate the change of momentum and kinetic energy of the projectiles at their target , but this is near to imposible , as there are unlimited scenarios with different results