Maybe I'm thinking too simplified here, but wouldn't this only work on the first iteration? After all the virus would replicate with Us in your cells and then the replicas wouldn't have the advantage anymore.
By that point the cell would be producing the associated protein though. Getting it inside the cell is the goal here from what I've read.
But that's what I mean, viruses need to replicate. They do this by injecting their RNA into your cells and hijacking your ribosomes for their replication. So the first viruses would definitely get past your immune system, but the replicated viruses would then be produced by your cells, so they wouldn't have the U->psi replacement that the first generation had. So every subsequent generation of the virus could be fought by your immune system. Effectively giving the virus a head start of one replication cycle. I'm guessing this wouldn't change much. But I'm not a biologist.