My experience with the older Gitzo standard spikes is that they are just about right and very effective...
In very hard ground, the thin profile gets them dug in easily.
In moderately soft ground, about the same. The tripod will sink to the end of the CF and stay there.
In very soft ground, for example, the edge of a marsh, the leg might want to keep going but in that soil nothing would help except something more cup shaped on top. Something like the Gitzo Big Foot. But that would not work as well as spikes in any dryer soil so unless you were planning on spending the day in a marsh it might not be a better overall solution.
I've never seen the need for the longer spikes, thinking that if the regular spikes are going to sink to the bottom of the leg and keep going, the soil 2 inches below isn't generally going to be any better and the end result would be the same.
I find that in practice, in a wide variety of soils, I see a significant difference in performance between the spikes and regular rubber feet, especially with my 500/4 working 700 or 850mm.
Tap tests of my GT3541LS with the rubber feet (stock or spike rubber feet add-ons) on a hard surface are generally up to 4-5 seconds, but in most soils with the spikes it runs more like 2-3 seconds.
That is almost a class difference (making appear to perform more like a Series 5 I recently tested with my lens). I think a basic set of spikes is the best accessory investment a long lens shooter can make.
Executive Summery: Don't worry, be happy... buy spikes
Edit: it is generally best to extend the bottom section at least an inch or two to protect the lowest collar from dirt and debris. That because the spikes generally do their job and sink fully into the ground with a modest downward push.