the shuttle valve controls kickdown shift feel by restricting how fast the fluid is released from the front clutchpack and the outside of the front servo.
So what effect would blocking it have? It sounds like it would make it drop down into kickdown a lot harder, but I also see the potential of a 3-2 flare by doing that also. If it were to relieve all pressure from the front clutch pack before the kickdown band had a chance.to apply it would flare like crazy.