At what point does carrying weight in the bed require a suspension upgrade?

[Schreibdave]

You can't legally increase the payload of any truck, nor should you try. Reducing sag borders on that if done incorrectly. The rated capacity is fixed. The only way to get more load carrying capacity is to get a truck rated at what you expect to haul. A 1/4-1/2 ton truck is never built to carry a one ton load without breaking the frame, and there are plenty of pictures to prove that point.
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Yea, I don't want that.

 

[John Kloehr]

For your question on ride quality.

A 2500 at full payload does ride about like a lighter-duty truck. The difference is more of maybe being firmer rather than stiffer.

One thing which greatly improves ride quality in an unloaded 2500 is airing down the tires. While 50F/80R might be right loaded, 45F/50R will be more comfortable when empty.

A couple caveats...

First, I kind of pulled these numbers out of my bum just now going by memory from my 2500 Chevy.

Second, check the ratings for your vehicle and your tires to see where the low safe point is, don't go below those numbers.

Third, never let air out a hot tire, air them down first thing in the morning when they are a cold as possible. Air them back up before putting a load on them.

The short answer is tire pressure makes a big difference in ride comfort when empty, and a big difference in control and safety when loaded.