The IRS lets the car handle so much better, you will kow the difference the first time you drive a GT-S(IRS) over that of a GT(solid alxe). Yes sence it is a 85 it would have the 7.5" diff, meaning most if not all of the Supra's(82-86) diff will fix.
Supra had LSD with a gear range of 3.73:1-4.30:1, while the Celica should a 3.91:1
but thats only on the 85 with the 7.5". You own a 84 right? then the first time you drive the 85 you will notice how much faster the car is and how the shift points come sooner. No Celica(82-85) came with a LSD, just the Supras.
I miss my 85GT-S