ARC side-mount's are supposedly rated at 300hp. In order to squeeze the most effiency out of a SMIC (hell, anything ATA for that matter), you need a proper duct to create a high pressure zone in front of the intercooler. The manufacturer probably takes this consideration into effect when rating a cooler for a specific power output, so you should as well.
Here's my ARC intercooler installed, which was fortunately found on eBay to the tune of $115 shipped:
As you can see, this specific model mounts at angle, reducing the length of the charge pipes to and from the cooler. However, this makes it even more essential to have a duct, lest the cooler do absolutely nothing due to the prohibitive nature of mounting the I/C this way.