What you need to do is use ACE to add passengers and cargo. Without them, your Zero Fuel Weight (ZFW) will be very low.
According to the aircraft.cfg the empty weight for the -300ER is 370,000 lbs, plus the weight of only the Captain and First Officer, which should be another 340 lbs. Add the fuel in the tanks, for a Gross Weight. The same applies to the -300.
This could explain the -300/-300ER ability to take off at such a low speed. ;)