Primary school children from English speaking homes who attend Welsh only schools are less likely to be top performers in maths, science and English than English-medium pupils
I know I know this is an anecdote and probably an exception to the rule, but I come from a solely-English speaking home, did first-language Welsh because I picked it up so quickly, and finished end of GCSEs top of my year in English with A*, got an A in A-Level English, did my Maths GCSE a year early and got an additional GCSE in year 11.
Being bilingual from a young age helped me to pick up other languages very quickly - A in GCSE French, conversational Spanish, even BSL - and so we're raising DD bilingually too (and have taught her sign language to compensate for the slight delay in communication often seen in bilingual children, but so far we've had no issue), and I'd be very unhappy if she wasn't able to attend a Welsh-medium school.