Sub NP with dual stab is an example of where not only does mega doom not need sun, sun will actually hurt it health wise(25% per sub, 12.5% for every turn in the sun). You can come in on something like ferro/forretress (if forre doesn't carry EQ), set up a sub as either the switch or do something else. Then, what they do after you set up a sub is when you decide to attack/set up. Mega doom is great in sun, but that doesn't mean it's useless outside of it. :]
Any further conversation we have should probably be taken to VM/PM :3
Edit: also, mega amp and mega doom fill different roles. Amp is a bulky attacker while doom is a fast attacker meant for sweeping. Mega ampharos only has one option for fast sweeping in the form of agility, while mega houndoom already has that speed, and can just set up for more power.