this just me but I have tried the ISMI 12.5, didnt care for it, I have tried the Variable 12 from Wolf and I could feel the difference between the Var and Conv but that only lasted for about 20 rds or so. So, in the end I just went back to using Wolf Conventional springs, thats it, Rather a 12lb or a 13lb in a 40 gun will fine.
All this was in a SV 40. Now, mind you, depending on the powder you are using matters a great deal on the weight of the spring you will use!
Ive used TG with a 180 Zero/1.180 OAL @ 4.5 grs and could never really use a 12lb spring in my SV pistol, just felt as if the slide was banging against the Grod.
Recently change over to WST and what a diffence! Same specs. to OAL abd bullets except now its at 5.2 grs of WST. In the same SV 40 gun I was able to lower the recoil spring to 12lbs and shoot the gun comfortably.
Now am using a Para 40 with STI mags same load of WST using a 13 lb Wolf conventional spring, I have not tried a 12lb only because Brownell and Speedshooters were out of them, I have some on backorder.
In the end, what Ive noticed is that there is a difference in feel and front sight tracking between a 12 and 13 lb spring but really what matters most is tracking your sights and shooting for points. When your pumped up, you can shoot that gun as fast it can go, no matter the spring.
Sometimes, having too light of a spring weight in the gun, I tend to or its just too easy to throw out shots without really aimming! Just a slight heavier weight in spring, gives me more a "up and down" to the front sight tracking and just that extra "milasecond" to prep the trigger and aim the shot, if all that makes any sense.
IMHO...