I replaced the mainspring on my 1006 with a Wolff 22# spring (factory is 18#) and have never had a failure of any kind, even with off the shelf ammo.
In my opinion, the stock springs are a bit light, as they come from S&W able to shoot even light factory loads. If you're handloading, chances are you're loads are in the hot range and a stronger spring really saves some wear & tear on your pistol.
I'm in the market for a 4006 and as soon as I find one, I'll replace the springs in it, as well.
Mike