Hey Lee,
Good questions...
Most 1500's are geared towards the beginers. because of the low cost the performance or voltage curve is a lot sharper then that of a high quality 2400 (made by Sanyo) I say Sanyo because it is possible to get "Generic" 2400's these would have a simular performance as your 1500's but with longer run time.
When you step up to a high quality pack ( made by GP or Sanyo) your car will have much more speed and torque due to the extra voltage and the discharge curve.
answers to your questions:
1) yes it is good to make sure you fully charge the pack before you use it. NI-CD's will develop a memory so it important to maximize that memmory effect by taking it through a complete charge/dishcharge. (while playing in your car)
2) That slow down is the "voltage curve" of your pack. your pack will peak high and then constantly get slower and slower. a high quality pack will have a more consistant curve. so basically the pack will have a constant speed until the pack dies. that constant speed will also be higher as well
3) your car is discharging the pack as you run it. it is best to run the car until a very slow speed. and then let the pack cool and re-use it or store it for the day. You don't want to leave a charge in your pack so if your car breaks or it starts to rain, then you will want to "discharge" the pack on a external device.
4) you can buy a cheap discharger from any LHS. I know Integy makes some good ones for not to much $$.
5) 2400's are still NI-CDs just with a higher capacity so yes follow the above with them and you should be okay.
If you get into racing thent he proceedures will change a bit but thats a different page... as long as you play you will be fine with the above.
I hope this helps. and if you have any other questions please ask away.
Korgae
SMC