I own a "93" Allante and it has the Northstar. Allantes were built from 1987-1993. There were three different engines and slight modifications along the way. The earlier models had the 170HP 4.1L V8, while the middle year models had the 200HP 4.5L V8. The 1993 was the only year model to get the Northstar. All are nice cars, but the pre1993 models had some potential for brake failure if not properly maintained. Prices range from a low of $3,500.00 to well over $30,000.00 for low mileage mint condition 1993 models. There are several differences in small items over the years, but the biggest was the engines and the change from Recaro's to Eldorado seats in the 1993 model. Also Cadillac switched from Bose to Delco in the 1993. I think they made a grave mistake in the changes, but I still love mine. I'm not sure of performance differences when compared to STS or SLS, but I'm sure there close.
The Allante debuted in 1987 and came standard with everything including the soft/hard top. In the later years, the hardtop became an option as well as the digital gauges. Of course in 1993, the Northstar was offered with the Road Sensing Suspension. Also in 93, the sound system was no longer BOSE and the seats were no longer made by Recaro. Other features pretty much remained the same. I think in 90, the CD player also became standard and so did Traction Control. If I remember right, horsepower started at 170 in 1987, went up to 200 in 1989, and then of course with the Northstar, went up to 295 in 1993.
Compared to the current STS, it really doesn't hold a candle except of course for the fact that it is a convertible. Compared to other Cadillacs, the car is alot lighter and to me, the ride wasn't as good. When I had mine, it also didn't seem to be as comfortable and of course since it was a 2-seater, there wasn't as much room.
It's a really nice car to have as an extra car to have fun on the weekends but I didn't like it as a daily driver.