Here is a recap of the fitting:
First set up consisted of H&R Super Sport springs (2" drop) and 215/40/18 tires (1.31" taller than stock). This resulted in rubbing. The H&R SS springs were swapped out for ST XTA coilovers. Slight rubbing persisted up front and the fender liner was trimmed back to provide extra clearance (no fender roll or pulling required). No more rubbing.
Tire sizing is a bit tedious with 18's to fit the ST W/8" width wheels. A 215/35/18 will trim down sidewall height .27" from stock. Overall height with the 215/35/18 increases .47" from stock. The owner went with the 215/40/18 trying not to decrease ride characteristics with shorter sidewalls of the 215/35/18. tire.
At this point, we know a few things. First, it is always a good idea to go with quality coilovers when increasing both diameter and width for the ST from 17x7 to 18x8. Drop springs do not adjust (obviously), so any issues with height cannot be eliminated without fender work. In general, a quality set of coilovers is a solid suspension upgrade over springs only. That being said, not everyone will need/want the adjustability coilovers offer. As reported, the 215/40/18's rubbed on 2" drop springs. Coilovers replaced the drop springs and trimming back the inner fender liner was all that was needed to finish the fitment. A 215/35/18 will allow for extra room to lower and likely eliminate the need to trim back the fender liner,
New fitments are always trial and error with lessons learned. We now have data to use thanks to this fellow ST owner. I love the 18's as they provide yet another change in appearance from the opposite end of the spectrum 15" wheels. Something for everyone makes the world go round.