Although it is easier to negotiate from a position of strength. If you are 300 points behind, there is not much incentive for another faction to ally themselves with you and declare war on the faction that is 300 points ahead of you, and 150 points ahead of the one you are asking.
On the other hand, if you're 300 points behind the leader and someone else is 150 points behind the leader, then an alliance between you and the guy 150 points behind the leader will at least bring the sum of your power ratings closer to the leader's power rating, and may even surpass the leader's power rating (if the leader had a power rating greater than 450). Alternatively, depending on the relative difference in power between me (lagging 300 points behind the leader) and you (lagging 150 points behind the leader), I might look like a tempting target for you to take over to reduce the gap between you and the leader - but if I'm at 300, you're at 450, and the leader is at 600, I should look like a strong enough target that you would consider not invading me to make up the difference in power, and I'm also a strong enough faction where I should be a viable ally for you to consider in a bid to challenge the leader. If this were something less close (say I'm at 30, you're at 180, and the leader is at 330), I'd be a very tempting target to take over in a bid to reduce the leader's lead, since I'm not strong enough (at least by power rating) without allies to challenge you in a war, and I'm also not strong enough to bring much to an alliance with you, unless by bringing me into an alliance you could secure my allies for your attempt against the leader. Absolute differences in power can be very misleading, and are a rather poor basis for the diplomatic modifiers.
I personally think that the diplomacy modifiers based off of power ratings should be based on the relative difference in strength rather than the absolute difference in strength (so, (my_strength - your_strength)/(my_strength) when determining my diplomatic modifiers towards you, and (your_strength - my_strength)/(your_strength) when determining your diplomatic modifiers towards me) since there at least should be huge differences between the difficulties of defeating a nation of power 30 when your power is 330, and defeating a nation of power 300 when your power is 600, and defeating a nation of power 3000 when your power is 3300. In the first situation I should completely dominate the lesser nation, in the second I should still be dominant, but it should take a fair amount of my strength (roughly half) since the opposing nation, while weaker than my own, is at least closer to being in the same weight class. In the final case given, I should not have wanted to go to war with that particular enemy unless I was certain that I could focus my Empire's full strength against that particular nation, or that the other surviving nations are so weak that they could not threaten me even in conjunction with my target. Strength 3000 versus strength 3300 should be a very tough war, with only small changes in the strategic situation having the potential to greatly tip the balance of power. If two such nations were to fight one another, the stronger should still need to put about 90% of its strength into the war effort if 100% of the strength of its opponent is involved.