With a proud spring like that, the first thing I'd look at is whether or not I could file down the kick on the blade's ricasso, instead of thinning the spring overall. The 'kick' is the protrusion of steel at the lower edge of the blade tang (ricasso), contacting the backspring's inside face when the blade is fully closed. That's what gives the blade a 'stop' to rest against, when it's closed. MIght post a picture of the knife with the blade open, to show if there's a 'kick' there at all, or how much it might be filed down. Also need to see how much room exists between the cutting edge and the backspring when the blade is closed. That'll spur some better advice on what may be done or what shouldn't be done.
Most whom file the kick will do so to lower a blade into the bladewell a tiny bit, to conceal a tip that's exposed above the liners or to lower the height of the closed blade's spine above the handle, giving it a slimmer carry profile. It all depends on how much kick is there in the first place and if there's room for the blade to lower without risk of the cutting edge contacting the inside face of the backspring. In most cases, filing the kick as such will cause the backspring to sink a little bit as well, when the blade is closed. If the kick can be filed down, it should be done VERY gradually, in very small increments, to avoid taking off too much and risking a collision between the blade's cutting edge and the inside face of the spring. Sometimes, just taking off a fraction of a mm will be enough to visibly lower the spring.
I'd not thin or narrow the spring itself though, as I'd think that could weaken it substantially. Might create other unwanted issues like very weak closing pull or causing the spring to break under stress.
