I would think the conversion rate for academy inductees to fully fledged professional players must be relatively low. In countries where football is a major sport probably 1 in a 1000 trialists will be admitted into an academy. Of those lucky enough to be I would suspect between 1 in 10 to 1 in 5 get senior contracts. The success rate would be higher at top clubs simply because they have huge resources and networks to draw on.
The conversion rate from a youth team player to a full time professional within the specific club is much much lower in top clubs. The bar is set so high in clubs such as Man U etc that very few youth team players make the step up through the system within their own club. However, youth team players at big clubs generally have a better chance of getting professional contracts with lower tier clubs. Having 'Manchester United youth team player' on your cv is not a bad start for aspiring footballers.
I'd say that on average, 1 out of 10 youth team players are offered a full time professional contract within their own club.
Middlesbrough has a great youth academy so Im in favour of the Nix one..it will churn out product from what we have to start with.