Accuracy trumps velocity with air rifles even for hunting.
Your 600fps certainly *may* work under *certain* sets of conditions.
If you have about 1000fps you're talking around 30 ft-Lbs. You have to have accuracy. But.. because we're talking low amounts of energy to begin with, you want every little bit you can get, especially if you're hunting in real-world conditions, such as some wind.
Why wouldn't you want every advantage you can squeeze out of your device/tool?
600fps is just gonna frustrate you at 40yds on a windy desert day. With my .22 Cal that does about 950fps real-world... even with that one... sometimes those longer shot Rabbits ya still gotta run up and give em another hit to end it quick. Because either they don't present you with the best angle, or it's just plain crazy windy and you're damn lucky it even hit 'em to begin with at that distance, what with the wind and all.
All I'm saying is... if he goes for around 1000fps real-world in .22 Cal air-rifle... he won't feel it's very lacking.
If we assume same 16gr pellet in .22 cal...
1000fps @16gr = 35.52 ft-Lbs
950fps @16gr = 32.05 ft-Lbs
755fps @16gr = 20.24 ft-Lbs
That's 15 ft-Lbs more than the 20!
For comparison purposes...
CCI Quiet-22 : 710fps @40gr = 44.76 ft-Lbs
which is 10 more ft-Lbs further still!
If ya went for the Crossman's Premier pellets that are 14.3gr...
755fps @14.3gr = 18.09 ft-Lbs