Recommended reading: "Pontiac Fiero 1984-1988." Brooklands Books, No date.
This is a compilation of magazine articles. I've casually looked through it to answer your question.
Car and Driver Feb. 1986. Fiero GT 86 1/2. Lateral G's in 300' skidpad: 0.80 g.
Road and Track No date. GT 86 1/2. 100' skidpad 0.83 g.
Road and Track Oct 1987 Formula. Skidpad not specified. 0.83 g.
Road and Track Sept. 1983 SE. 100' skidpad 0.812 g.
Car and Driver Sept. 1983 "Pilot-built car"
282' skidpad 0.81 g.
Its interesting to me that both the four and six had the same lateral acceleration. I feel the four is better handling because of it's lighter weight, typical of lighter cars.
What has been the experience of you auto-crossers?