I did a fair bit of experimenting with gas strut conversions some time ago. My overall opinion is that it is worth the extra cost.
The gas strut itself has more energy than the compressed air volume is capable of generating so you will get every fps that you can possibly get. Expect something around 730fps in a .22.
One of the most noticeable things is the lock (firing cycle) time becomes incredibly quick. So you'll see an overall increase in accuracy just because of this.
As well the gas struts have a substantial increase in rearward recoil. And also an increase in muzzle report.