In both cases (running stock or with Deavers) you will see a slight droop in the bed under load but nothing crazy. In all actuality you probably would not be able to tell the difference between the two and if you could it would be minimal.
Where the real difference comes in, is the stock springs have a load spring (the shortest spring on the bottom of the leaf pack) which is better equipped to handle load for long periods of time.
Deaver +2s and National SDs are not designed to hold more than 250 to 300lbs in the bed for a long period of time. i sacked out a set of +2s in less than 5 months because i was running a chase rack which was around 500-600lbs 24/7.
Deaver is a great company and they took care of me! I had the springs reached and they added a load leaf for my specific needs.
If you are going to be running any type of load over 300lbs for long periods of time, get a set of Deaver +3s or National HDs.
You can also always give us a call, tell us what your specific needs are and we can have a custom set made for your specific needs.
Hope this is helpful. Any other questions feel free to shoot me a PM or give us a call.