OEM size for a 2000 SVT is 215/50R16. A 215/55R16 tire is 0.8" larger in diameter, not 0.4". A 215/55R16 compared to a 215/50R16 will provide a slight decrease in steering response and ultimate grip. Will it be noticeable? Yes to some, no to others.
I think I figured it out. You mean you have fitted 215/55R15 vs the OEM 205/60R15 tires than many Mystiques and Contours come with. Only SVT Contours come with 16" rims, which are largely less common. Based on that assumption:
205/60R15 = 24.7" diameter
215/55R15 = 24.3" diameter
There is the 0.4" difference that you probably installed on many Contiques. I would probably recommend a similar size for those running the 15" factory rims.