The expression "X is not so Y" must trip of the tongue of English-speaking Scots, but it's not that common south of the border, except in the extended construction "X is not so Y as Z" (Stornoway is no so busy (or "as busy") as London). More likely would be "X is not that busy", "X is not too busy" or "X is not very busy" in stand-alone situations where you are not making an explicit comparison. The analogous Irish construction might perhaps be "chomh gnóthach le".
I have also heard it with a very definite hard c sound at the end. This one has a very subtle g.
I used to live in New Mexico. There were two towns there that had their own unique take on how there town was pronounced. Madrid was MAdrid and not MaDRID. And Thoreau was pronounced "throo"