Maybe he's a v8 head and have NFI what sort of power these pieces of jap crap make...
By all means 250fwkw is perfectly fine for a slightly modified RB20 and only just rb25det.
440cfm aint a great deal, its definitely not rwkw, its flywheel kw.
Do a little research in to airflow and its power correlation, do your self a favour and don't trust other people to make the correct decision for you.
People are stupid. Do some research yourself, ask around.