The gas spring is a nice upgrade for a springer. Pre-load doesn't work the same as a steel spring. The load-displacement curve for a gas spring is almost constant for the initial compression, then it increases slowly. Here is a graph:
Gasspring.jpg
So you get a more even release of energy on firing - but cocking effort is also greater.
I have ordered a bunch of gas springs to play with too:
http://www.dadco.net/Catalog%20PDF/micro_catalog.pdf