Many people say different things because there have been many beliefs spread as fact for many years and it's hard to uproot those beliefs.
Testing this particular question is also very difficult due to client-server latencies. It's impossible to know when a spell actually starts and when it finishes. It goes like this:
1. You hit a macro or cast a spell from the menu. Your client sends a message to the FFXI server saying "I'm casting this."
2. A fraction of a second later, the server receives this message and initiates casting. This is when the spell officially starts. The server sends a message to your client and all players within 30 yalms of you that you have started casting.
3. A fraction of a second later, you and everyone within 30 yalms of you receives the message from the server that you started casting. The delay in this step and the previous one cause the slight delay between when you press your macro and when the spell begins to cast on your screen. The more lag you have, the longer this delay.
4. When the spell finishes, the server sends another message to you and everyone around you that the spell has finished.
5. A fraction of a second later, you and everyone else receive the server message that the cast finished.
Because of those delays between when things actually happen and when you receive the information that they happened, it's impossible to measure small changes in cast time, such as the effect from an item that adds 2% fast cast.
However, my own testing which I posted in that thread has been successful in at least showing that there is no cap at 50%. You can definitely go beyond 50%. No cap has yet been proven. Beyond that, we have only a vague understanding of how fast cast & related stats work.
I just recently hit a wall where my Cure casts were finishing before my <wait 1> processed, and so my potency gear wasn't swapping in time. This means my Cures were casting in less than 1 second, where base time is 2.5 seconds. So that's further proof you can break 50%, since 1 is 40% of 2.5. I ended up tossing the Winged Wand in my satchel for now until I can figure out what to do about that, and it brought my Cures back over the 1 second mark and my potency gear is working again.