A couple of things to add:

While W50 is stronger the ratios are only marginally better than a 60 series transmission. Depending on how hard the A14 is tuned it won't improve the lap times (granted if you are breaking gearboxes than yes it's worth it.) The later W57-58 have ratios similar to the 56 series mid close ratio 4 speed.

I run the 56 series close ratio 4 speed behind the A15 in my car (it's making around 100 whp) and they last about 4 seasons before they strip the teeth off 3rd on the cluster gear. They might go longer but I pound the curbs and that can't be helping. Since 2003 I've broken 3 cluster gears, I run 6 events a year , so figure I'm getting 20-24 events. The boxes get kind for loose about 3 events before they fail. I've now accepted that I need to service the boxes every couple of years, no clue if this will help the reliability but they will certainly shift better.

I have found a place in New Zealand to repair the clusters at a reasonable price so I may stick with them. I have a stock 60 series 5 speed but obviously it has the big gap between 2nd and 3rd.

Tom