85 GT's were definitely not 15.9, mine(long gone, sigh) went 14.49 in HRP with just a K&N filter. 82K miles and never touched the timing, mufflers, etc... My first car didn't know about modding.
86's were solid high 14 sec cars. In poor tune would go in the 15's.
My current 90 LX 5.0 convertible has run consistent 14.5's(10 passes) non slower than 14.59 with just a K&N, and pulleys. I've added a MAC prochamber but haven't gone back to the trap.
So if you plan on racing the GT, it should be real close.