Why do I see some high performance engines w/ breather caps on the oil spout??????

[84LX89GT]

The GT i just bought has a K&N breather on where the oil filler cap used to be and i've driven it hard and haven't had any oil spillage problems. The guy who had it before me also routed a line from the throttle body to the oil filler spout like it was stock. I think it may be causing a vacuum leak large enough to bother the computer because when it's cold it has a surging idle and almost dies and another thing that annoys me is that i can hear a whistle whenever i give it gas or let off because of the vacuum through the breather. It seems to work well at getting rid of the gasses, though, but i'm eventually going to put it the way it is stock.

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previous name: 84stangLX
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT