My 1990 Taurus SHO was one of my sleepers. 99.9% of people had no clue, because it looked like a Taurus. A pain to maintain with valve adjustments every 60,000 miles. The rear bank was a bear to get to. Did a ton of performance modifications. It handled amazingly well on the track. 3 series BMW’s were toast for about 6 years. I put 330,000 miles on that car (2 engines), and was still not afraid of it. Yamaha set that motor up so that the power band hit from 3,500 to 8,000 RPM. At 270,000 miles I took it out to Texas Motor Speedway, put it on the road course which included the oval, and ran it on 110 octane gas (that was the only way it would stay cool) for 2 hours between 3,500 and 8,000 RPM. Downside was brakes. Later models had better brakes and I upgraded, but they were still shredded at the end of that run. I managed to drive it home, put it up on jack stands, and order brake parts.
I have had 2 4-door CTS Vs, They are sleepers too but their aerodynamics give them away to more people. I still have number 2, a 2010 with 556 hp/552 tq, with 178,000 miles and am still not afraid to nail it.
Unbelievable motors in all three.