Haha, actually i have a similar setup.
my is HB12x (t-bar) with softpack 7.4v, 4000mAh raised to clear the t-bar.
i find this setup very good since the weight is similar to 4NiMh so this makes the car very stable.
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?
yes, the battery pack is moved forward compared to nimh position. the car may 'push' more. to add more rear grip, just use softer rear tires or softer damper springs.