I remember the weight difference between a 3 and 5 series of that time to be marginal.
Even if it was within 150lbs, a 325
xi with a few cinder blocks in the trunk is going to be a pig.
EDIT: 07 525xi is 218hp / 184tq and 3703 lbs according to wikipedia. Tested to 100km/h in 8.4 sec.