Well, without any oil in the center diff, all the power will be transferred to the end with the least traction, which is the front end (-> as all the weight is transferred to the rear upon acceleration, it has the most tration).
The more power that is transferred to the front, the more the frontend will pull the car around, smoother and easier to drive generally.
Lets say we have a starting point of 7000 oil in the center diff.
Moving up to 10k will result in more power being transferred (or forced) to the rearend, and the rearend pushing the front (livelier, more agressive)
Moving down to 5k will result in more power being transferred to the frontend, and the frontend pulling the rearend around ---> which is what you want, right?
So from whatever oil you've got now, try dropping down 1 or 2k depending on how much pull you want and see how it goes.
As for which pipe for lowend, well from what i've tried the RB 063 did a pretty killer job, but it isn't too durable. I've also heard the ofna copies of the 063 and 086 don't perform quite as well as the originals, so might be best to steer away from those too. I run an OS T-2050 right now and it seems to match my old 063 in lowend (or very close anyway) and is built way tougher, but the pricetag reaches into triple digits.
I think if you want to milk more lowend from the 8 port though, the clutch might be a good place to start, might be worth trying some stiffer springs (1.1mm) and lightening your clutch shoes.