5th is usually oil starvation due to the box design lacking decent oil feeds to both gears.
1st-3rd generally break with a large amount of power/torque with traction (slicks, LSD etc), final drive bolts also a favorite. A resaon I wanted to try a Quailfe LSD instead of a plate was to increase wheelspin and lift load off the gears... I broke CV joints instead.
I'd guess the 085 box is validated for 150Nm or there abouts, 02J is 250Nm, 02M is 350Nm, I'm in the process of fitting an 02J, due to the 085 being far below what's required for good power and track abuse, as lots of people are now finding!