They share the exact same engine and drivetrain. The only big difference is the '86 has a chrome grill with "BUICK" at the top, while the '87 has an all black one with "BUICK" in small letters on the driver's side.
'86 is rated at 235 HP and '87 is 245 HP. But that was because the Corvette HP rating went up to 240 in '87. Even though they are rated different, both years perform the same.