Do you hunt with your airguns or just shoot targets? If you only shoot targets less power will gain more accuracy. At the distance you're shooting drop won't be an issue and less recoil makes a gun easier to control.
I have one Ruger Air Magnum and two Diana 350's. The R.A.M. still has the factory spring, one D350 has a pg3 kit (.128 wire) and the other has a Vortek D48 spring. The D350 with the D48 spring gets shot the most, the R.A.M. gets shot the least. The one D350 was used as a test rig with a (Vortek) 38 coil .135 spring which produced very impressive power (23fpe in .177), but loosened every bolt/screw on the gun while shooting. Would be better in a .22 cal gun, but threadlocker would be needed. This spring has more "gusto" than the standard spring and is about as heavy as one can go without the gun destroying itself. Using a Vortek D48/54 replacement spring in a XS28/RAM/D350 makes for a easy to cock/very mild shot cycle and still makes ~15fpe (in .177). I find it's more enjoyable and much milder than a XS25/Blackhawk/D34 at full power (15-ish fpe). The PG3 kit (.128 wire) is nice and offers some improvement compared to a factory guide with a drop in spring and a bit of moly paste. I was getting 17fpe in .177 if I remember correctly (after installing a thick washer). Cocking effort was reduced but the gun also lost some power. Watch out for the cocking shoe dragging on the spring guide, you may need to lube with some moly paste to reduce scoring. Mine (cocking shoe) has made a groove in the guide and needed some metal flakes cleaned out of compression chamber and piston. After cleaning and moly paste application I've had no issues, just made sure to line the scored in groove with the cocking shoe. You may not have this issue and may have been specific to my gun. There were two D350 (and clone) kits available at the time I purchased, a H.O. (.128 wire) and a S.H.O. (.135 wire) either should handle hunting duty, especially in .22. For 25 yards or less I use a ~9fpe D34 which is amazingly accurate. You don't need a lot of power, just make sure placement is good. Any XS28/RAM/D350 making decent power should meet your needs and then some. Though I feel the effects of high volume shooting at full power also. Usually limit myself to 30 shots or so.Take it easy, hope you enjoy the PG3 kit.
I'm not familiar with the B40, I'll have to look it up. My D34 has one of the Vortek custom cut springs, started out with about 10-10.5fpe similar to your XS25, but has settled since then. I'll probably keep shooting with that spring until the velocity drops below the mid 600's as I don't think it will ever break. I'm a big fan of Vortek springs, that's awesome that you got to test a bunch out. The XS28/D350 is great in that it doesn't need to be high sprung to make decent power. When you push the power limits they can get up there with (or close to) the Turkish magnums, but it's hard on these guns. Once dialed down they really shine in my opinion. My R.A.M. shoots .177 H&N Baracuda Hunters out really accurately/fast/flat so I've kept it stock for that reason. When the factory spring breaks the Vortek .135 38 coil spring is going in, I just like to keep a high power .177 for smiles and grins, or for those rare occasions when you need a 20+ fpe .177. Now I sound conflicted, but I like these guns at all power levels, just enjoy shooting the mild ones in higher volume. A side benefit to mild power levels is not needing 10+ grain pellets to stay subsonic in .177. Something I never thought about until pellet options started to thin out a bit.
Found this :https://www.gatewaytoairguns.org/GTA/index.php?topic=390.msg2487#msg2487And B40:https://www.gatewaytoairguns.org/GTA/index.php?topic=405.msg2489#msg2489