For example, If I used a Nox weapon with a damage of 950 and on average I hit 5,000's per hit on enemy's not taking into consideration of elemental bonus or crits... How much higher would my average hits be if I used per say a Nemesis/Slave weapon with a power of 1,400?

Would it be even worth paying for these items that are several millions more than something much cheaper that works? Do these weapons do so much more damage, like 30-40k+ on hit as I upgrade them compare to Nox that it would be silly not using them?

Not an expert, but this video explains at least that nox isn't well suited for current and future end game.

https://www.youtube.com/watch?v=K2ZQwe4FmGw

As for plain damage, again, not an expert, but we could say that other than the plain attack power of a weapon, whatever you have affixed and the potentials you unlock from the weapon may be as, or more important than that plain number. Some affixes/potentials can give straight percentage increases to damage. For example, I replaced my nox rifle on my ranger with an elder rifle. The new rifle gives me a potential that makes my attacks do a fairly nice +25% damage when moving, which is perfect for ots view. I assume higher tier weapons may have even better potentials. Also when comparing with nox, some high tier weapons can go a few hundred more atp, and that's quite a bump in dps.

http://4rt.info/psod/ is pretty accurate. Don't mind the class modifiers much since you're comparing weapons and not class combos, but ticking boxes like PP slayer makes it more accurate since those give attack. You can mostly leave enemy values at default.

@LovedChunk122

Divide your total attack (weapon element damage isn't shown on your stat screen by the way; it's element% * your weapon attack) by 5 then multiply whatever number you get by ALL of the multipliers you may have (PA, JA, skill trees, weapon potential, etc) and that's your rough damage increase.