Is it worth it to provide "lesser scoped" interface of the same instance, if its be "hacked" via casting?

interface ReadFooRepository {
	fun getFoos() : List<Foo>
}

class FooRepository : ReadFooRepository {
	override fun getFoos() ...
	fun saveFoo(foo: Foo) ...
}

@provides @singleton fooRepository() : FooRepository = FooRepository()
@provides readFooRepository(fooRepository: FooRepository) : ReadFooRepository = fooRepository

class FooViewModel(private val readFooRepository : ReadFooRepository)
 ... only consume values

Kotlin does it with List

hmm true, but arent they doing it for perf reasons?