Many people have reported successful use of 87 octane fuel in past Northstars (post 2000), but to be totally honest with you, I havent seen many using it on the newest ones. The Northstar engine is a complicated and technologically advanced powertrain and I tell you what, if I could afford an XLR... 87 octane gas would not go in it...
Just my opinion but... "Use at Own Risk" comes to mind.
Modern engines have the ability to adapt to lower grade fuel because of knock sensors and VVT. They just retard the timing until your low grade fuel stops pinging. You're just lowering the performance potential of the engine by using lower grade fuel as retarded timing = lower performance.
Still... the manufacturer does not put a premium fuel requirement in the manual just for the hell of it. In fact, this is more to their disadvantage as some might not consider it with the high cost of fuel. Like anything else, Im sure its ok, but over the long term could have implications. Even when Ive been made fun of by friends and co-workers for putting cheaper fuel in expensive cars that I have purchased used, its STILL at least MID-Grade. In other words, its really kind of silly to put cheap gas in a 70k car, especially if your in it for the long haul.