[wparsons]

Originally Posted by Senzai

The single biggest improvement from stock is upgrading to 18" wheels and appropriate tires for your driving application (road, track day, time trial/competitive, etc). This has been validated on the track with lap time data (and posted elsewhere so I won't repost here). Key here is not to increase the weight of the wheel substantially over stock as increasing unsprung weight will degrade suspension performance. Our stock wheels weight ~20.1lbs. I upgraded to a set of 18x8" wheels that weight the same as stock so any increase in unsprung wieght would be due to a bigger tire.

Aside from the cosmetic benefits (depending on your tastes), you need to approach changes in springs as it relates to the overall suspension setup and your goal (increased performance or enhanced look without compromising ride compliance, etc). For me I did springs after I uprated wheels to 18.8/235/35, installed Magnaflow CATBack, and dropped in a K&N filter. I'm keeping my OEM wheels in the event a do some spec FR-S racing in the future.

An 18" wheel will absolutely not make you faster on a track, sorry. Same width/compound tire a 17" will be faster because the weight is closer to the hub (the barrel of the wheel is the heaviest part).