Interesting that you are running 4000 mah! The placement of my battery is slightly more forward than the centre line of original battery position for 4 x NiMh. The 2800 mah should weigh less than than 4 x NiMh. That's why I wonder if that causes the car to have less rear grip.
I raise the battery up by about 4mm to clear the T-Bar. Had to remove the carbon T-Bar brace otherwise the battery will need to be raised by 1cm! How about you C-hing?