I guess the question is what are you really trying to accomplish. If its saturation, there are some good pointers above. If its actual range extension, then yes it can be valid and successful.
I had a previous game where I was trying (desperately) to stay outside the active sensor range of an NPR whos tech was MUCH better than mine. I was keeping him from slaughtering me by having faster ships and ECM, but his weapons tech and PD tech was much better. I did have an advantage in that my sensor tech was better, and my detection ranges were about 20% or so better than the Bad Guy.
To leverage the advantage, I built two missile designs. The first one was a size 6 missile that has a fair amount of fuel, but just enough engines to get it down range faster than either his ships or mine. It was built for endurance. It ran size 6, 1pt in engines and 2pts in fuel. Had a range of about 100m k/m and speed was like 13k km or 14k km. It was packing a single size 3 missile that ran 40,000km, a 4pt warhead, and a half point of armor, with a range of 5m km. I would lob these at the bad guys from 100m out, and then the second stage would seperate at 4m km, well outside of the AMM range of the bad guys. Having the half point of armor made the missiles much more survivable.
The design worked pretty well for what it was intended to do, which was to give me a shooting solution at a range outside of the bad guys. It also fit existing launchers in the fleet, so that solved any issues around refit.
I had a larger size 8 design that was simiar, it fired a size 4 missile that either had a thermal seeker or an active warhead that ranged out to 125m km.
You wont saturate targets like you can with multi-warhead missiles, but if your looking for very long range to get outside return fire, they work well.
This also works very well if your going up against higher tech enemies. 2-stage missiles with a 2nd stage with a high speed armored missile are a good way to get in hits against higher tech PD (as long as it isnt Mesons).