I've crossed the border at Detroit/Windsor with my 03 a total of 3 times. Here's each...
1st time - going to Canada: Brand new, still had temp sticker on inside of window. Canadian customs asked where is the car registered. I showed all the paperwork. They let me go no problem. I was with a female.
2nd time - going back to US: US plate came and I had it sent to Montreal. US customs asked what year and how much I paid (which I was not inclined to answer but did anyway). He then asked about the STI since he was considering getting 1. We talked shop for a few minutes and he let me go. I was with a female.
3rd time - going to Canada: Car now mod'ed. Before getting to Canadian customs even, it was US customs (remember I'm heading to Canada now) that pulled me over. 7 agents stopped me right after the toll booth and inquired about my joy and how I can afford such a car. No searches just questions was all. They said to keep the speed down. I was with my younger brother.
Interestingly enough in the Spring of this year I drove my S2000 back from Montreal to the US. Crossing the same border, I was pulled over. 6 US agents searched the car inside and out. 1 agent asked me to open the airbox and asked "Is that a modified or stock filter?" I said, "Stock." He said, "How come you don't get a cold air intake?" I replied, "Why are you wasting my time? I'm tired and just want to get home." They reviewed all the papers and my Canadian issued frequent visitor pass and let me go. I was with my younger brother at that time too.
I conclude - if you drive a hot car with a hot girl, they let you go. If 2 guys with shaved heads, they pull you over. Hmmm...profiling maybe? Who knows.