I have a 440b, and I run a 200 lb counterweight, for a couple of reasons:
1) The engine that I have repowered it with (Chonda) is much lighter than the original Koehler.
2) It does help with the lifting, especially when fitted with a heavier attachment, like a grapple bucket or a 4-in-1 bucket.
200lb's is about the limit that I could add as it makes the front end very light, especially when moving the machine around with no bucket on it. In fact there are times that I drop one weight off for better stability.
I am pretty sure that the nose weight on your machine is there to offset the weight of the engine. When the 4XX series first came out, they were fitted with a much smaller diesel engine than the one that you have. With an empty bucket, your machine is intended to have a 30/70 weight distribution front to rear. With a full bucket, that distribution reverses.
I considered getting a 463/S70 counterweight for my 440b to give it more down pressure when digging. Then I installed a set of teeth on my bucket - problem solved.
And with regards to how much your machine will lift, there is no doubt in my mind it will already lift 1000 lbs. The 700 lb rating is based on one half of the tipping load. If you can get the back end light, then you are well past 1000 lbs. Just be careful!!
Cheers - SR