So, a final update.. I switched out that one leadstone conduit for a redstone conduit and yes it did fix things.
However, when I first put it in I saw the full leadstone energy cell reporting 80RF/t in and 26RF/t out.. I was worried, but after about 30seconds it all settled out and it's 26RF/t on both sides.
So perhaps the conduit between the cell & ME system was never able to fill? (maybe the ME system was draining it to fill it's own internal buffer too fast?) I know Hardened conduits have a 2400RF buffer, but Redstone Conduits don't (they just have a flat 5% loss, right?) .. Either way, it's working now.