hi how your build coming on , tbh honest mate i dont think its a very fair comparison as the cosworth yb was designed and built with motorsport in mind and as weve seen massively over engineered for big bhp . back then there was no emission laws to contend with in building a production engine etc etc so it was an open book to do what you wanted in terms of engine design.
the zetec engine had to meet strict emission laws and the cylinder head was redesigned to comply with this ( hence look on syzbk thread and you will see the first development head for the zetec and a standard head and the ports are huge) thats why they have trouble flowing big numbers as it was simply not ever designed for motorsports use , just a mass produced emissions friendly engine.
now to get more power and stay on the good side of emission laws like the st170 engine the cylinder head was reworked to give more power.
i personally think with a standard production casting zetec engine that 600+ bhp is achievable with reliability and the only person who will ever see more on pump fuel will be the man that ive wrote about above (800+ bhp)
i also dont agree that a 500bhp zetec cost the same as a 500bhp cosworth as you defo dont pay the cossie tax for starters lol and if you look around hard enough and get stuck in you can save yourself a shed load of cash.
my first 522bhp cost around £7500 inc vat with mapping and an engine dyno session , everything custom made etc , i bet a cossie engine would cost that from a decent tuner with no inlets etc
i think the zetec is far better value for money imo as zetec parts are just far cheaper and you can pick a working engine for £100

as for the future who knows but i personally think its only going to get worse in terms of the dreaded emission laws and you will see the rise of the smaller 1.6 turbo coming back

cheers paul