One of the many reasons I dislike the FB" nomenclature is that it makes it sound like all 81-85 models are the same
There are 2 types of axles during the relevant period: 24 spline axles and 26 spline axles.
84 to 85 models in the UK and Australia (what we call series 3) got the 26 spline axles with open diff centre 3.909 ratio.
81 to 83 models in the UK and Australia (what we call series 2) got the 24 spline axles with open diff centre 3.9 ratio
In the LHD market it all changes.
84-85 models in the US and Canada came with different specification levels equipped with different wheels and brakes. The base models got the earlier style axles with open diff centre, smaller brakes and 4x110 stud pattern 13" wheels. The GSL and GSL-SE got the 26 spline axles with bigger brakes and 4x114.3 stud pattern 14" wheels. The GSLSE also got a LSD with 4.1 ratio.
The 24 spline axles are average.
The 26 spline axles can take a bit of abuse. Once you have a sturdy box and diff centre, however, the axles become the weak point in the chain. An acquaintance of mine from way back had his series 3 with 12AT running down into the low 10 second quarter mile. On the way to getting this fast, he had to upgrade to a s5 turbo box and Torsen LSD centre. But in the 10s he began breaking axles on a pretty regular basis. Now this is hard launching on the drag strip with sticky tyres.
An 11-12 second car is still FAST and the 26 spline axles should be able to cope with on street tyres. Even with 400rwhp you will be struggling to get traction before you need to worry too much about axles. If you get some grip on those tyres however, different story.