Di Pasquale, Grosjean, Gasquet and Monfils as year-end #1, plus Jeanpierre, Mathieu, Mahut, Morel, Tsonga, Chardy, Sidorenko won a GS title each in recent years.. seriously? If anything, they've been under-performing after going pro.
you confused the thread question. it's asking which countries players underpreform once they go PRO after having great junior results
as you just stated, it's obviously France because their players often dominate juniors only to fail at pro. of course fail is subjective because some have had good top 10 success but none have been able to be top dogs like they were in their junior career