counting lbs, is when your drag racing. Albeit, everything counts in small amounts, like the Depech Mode song goes, but I digressed.
ARC makes the lightest IC cores, Spearco/Kelly Aerospace cores are a little heavier but not by much. The side tanks also play into this weight calcualtion. But the total difference between any of the ICs is only a few pounds. You can take a dump before racing and make up the difference. 0.100s are lost by losing 100s of pounds.