I didn't know that this was called telescoping but I agree that Maths is one of the easiest subject areas to do this with - the material is very well defined for each grade level. My DD8 has been using the Singapore Math + First in Math and we are about to review the 4th and 5th grade stuff via the 'Challenging Word Problems' published now by Marshall Cavendish.
We don't 'hothouse' per se - we just give her the books to work through in her own time and she taken this in her stride outside of regular school. I was concerned that perhaps she was going too fast to truly internalize some of the concepts (she was nailing the exercises in the books but even so...) I am expecting the challenging word problems to test that.
My $0.02 is that you should try it - I don't see that it will do any harm.
