An Escos is a shortened Sierra floor pan and running gear, yes.
I can't really see where your coming from though tbh, it's only the same as saying a Sierra Cosworth is just a Sierra underneath, you pay for the rareity, legacy, not to mention the Escos was the rarest model of the lot, hand built by Karmann Ghia.
Being you had Cosworths before you will appreciate just how rare a standard example is, but then seem as you want more power then you arn't looking for a standard one anyway.
The Focus ST will be a better everyday option, a Cosworth isn't a everyday car really, by that I mean the ST you can park without worrying if it will be there when you get back.
Martin