Aside from the 20hp, there is no other performance advantage. I put the $800 Cobb AP in my car and had probably 30-40 more hp than an IS. That being said, I would still get the IS for the wheels and exhaust. If it was between the N54 and N55is, I would go N54 all day.
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2014 CC R-Line DSG | 2010 S4 Sold | 2010 BMW 135i Retired | 2006 Lotus Exige Sold