Ground Effect fit is pretty spot on- I have one of their Ice Queen Jerseys in size large and yes, it is designed to go over clothing.

I would say go down a size if you want something to be tight fitting.

About 10 people in my cycle club wear She Shells and Storm Troopers (all orange) and swear by them. My Ice Queen is really well made, does what it says it does and has lasted heaps of washes.