Here's a figure showing the percent change in MAF readings for 6 different aftermarket intakes versus the stock intake. The K&N IS one of those being plotted here.
If you think a 10%+ lean shift at WOT is fine, then more power to you, but I don't, and I wouldn't go around telling other people that they don't need a tune for it either. The Perrin intake is also one of those being plotted here, it's actually SMALLER diameter that stock, yet it too has a 10%+ lean shift at WOT. Just the other day one of the Perrin reps posted in a thread in the Newbie forum and backed up that statement, the Perrin intake runs 10% lean at WOT on the stock tune. They are of the opinion that a 10% shift is not enough to warrant concern since the stock tune runs so rich anyway, but I don't agree with that sentiment.