To my knowledge the e39 does not have any steering upgrade.
The 540i has recirculating ball -type steering and the 528i has rack and pinion.
Drive cars with both types and you will see a big difference in each.
Personally I think the rack and pinion provides better handling response.
My understanding of recirculating ball on the M5 and 540i was due to added front end weight of the V8.
The recirculating ball in the V8 cars is not to be confused with an "upgraded" steering feel...
The primary changes in the e39 cars were in '99 where the engine was technically updated, hence the '99 cars have m52tu and m62tu designation vs. m52 and m62 prior to '99