I run a 22lb Wolff recoil spring for the full power loads I shoot from Double Tap or my own reloads. I have shot these loads from a 17lb stock spring, and it will work. However, if you utilize a stock spring with full power loads, and chrono it, you will have a wide deviation of velocities. The reason being, with such pressure from the full power loads, the slide will begin to unlock several milliseconds premature, resulting in the spread of velocities.
It would only have to stand, if the slide begins to unlock premature, the stock spring would need beefed up a bit. With that being said, I have run normal "weak" 10mm Federal loads through the gun with the stock 22lb spring and cycles just fine. As mentioned, the 22lb spring is slightly more tough to cycle by hand when compared to the 17lb spring.
Here is my two cents, while I do not typically qualify to carry my G20 on duty, I would run the stock spring for duty or everyday carry with full power loads. The thought being the easier to manipulate spring would facilitate the many alternate methods of charging the gun should an arm be taken out in a fight and a reload be needed. A few (if not numerous) full power loads in an emergency will not hurt the gun and I would place the stronger spring in for target shooting when the added muscle to work the slide is of no concern.