4 speed ka-booom !

The brass rings may appear to ride against that surface, but in fact, they park against the little struts, or rather float there. The slider, has the struts pressed up into it by the energizer spring. When you initiate a shift the slider carries the struts forward which then push the ring forward against the coneclutch of a gear. This squeezes out the oil and begins the braking action, synchronizing the speeds between the two. When the speeds are reasonably close the slider overpowers the strut spring, and slides all the way forward and over the clutch teeth.The points on the teeth help to guide the slider over, during the final travel. They are now locked up and the shift is complete. The entire procedure takes but a moment, and depends on the brass braking the cone, and the energizer springs pressing the struts into the slider. If either are lazy, the shift will be slow, sloppy or grindy. I always marvel at how well that system works.

If you drop the hub into the slider, on the bench, you will see that the area that you have outlined with a wedge cut, only comes close to the slider. I have never cut that area in any box, and my trannys shift like lightning, as high as 7200(or more).I'm not saying it won't help, I just never saw the need for it. IMO it would be more beneficial to ensure the slider doesn't stick to the hub, as it's moving forward. If you examine the hub, you will see all the wear marks that the slider has made on it, from transmitting torque through it, from the slider to the mainshaft. Those areas I cut or poish, as may be required. Remember that during this sliding action, the clutch is disengaged and from the sliders viewpoint the tranny is in neutral.There is no torque being transmitted at this time. However, if this area is smoothed the shift effort may be decreased. Which is important if you have a really short stick like I have,(don't even think it),or you are a girl.
For street driving, you can't beat the H-pattern shifter. I tried the V-gate;at the track it was dynomite. But on the street, back-shifting was such a pain, literally, I pulled that thing, and it's been on the shelf ever since. And yes, I reworked that thing every which way possible to improve it's workings. In the end, I went back to what I knew worked.
There are way more important other areas to cut.