[SigmundA]

The check is added if it sees a varchar column and nvarchar parameter predicate on it.

It currently just does a scan in that situation which orders of magnitude more expensive with a cast for every row vs a single extra cast check on the single parameter value that may avoid all those other casts in a common situation.

There is no planning overhead, it's already detecting the situation. The execution overhead is a single extra cast on top of the cast per row, so n+1 vs n with the potential to eliminate n with a very common charset.