found out the problem... with 215/40/18, the diameter of the tire is 1" bigger than stock. without going to a coilover system like natey, the tires actually hit the spring perch. going with a 35 series tire will reduce the overall diameter by almost a full inch.
edit:
adding exact numbers for comparison...
stock 16" tire diameter: 23.7"
215/35/18 tire diameter: 23.9"
215/40/18 tire diameter: 24.8"
and just for kicks in case anyone wants to know
215/40/17 tire diameter: 23.8"
205/45/17 tire diameter: 24.3"
205/40/17 tire diameter: 23.5"
all calculations based on
http://www.miata.net/garage/tirecalc.html and verified by daveB using his weird math techniques.
just to note, this is for a 18x7.5 +45 offset wheel.