I will confess - I just upgraded to Transparent Reference interconnects and speaker cables. And I do remember years ago, prior to my first Transparent cable purchase (Super interconnects and speaker cables), thinking that this was a ridiculous amount of money for cables. But I had listened to a complete system I was buying, and decided to go with everything I had listened to, including the expensive cables.
A year or two later, I tried the Transparent Ultras, and they were a shockingly big improvement. So I upgraded (with full trade-in value).
This upgrade, to Transparent Reference, was because I also upgraded to Rockport Avoir IIs. I upgraded the cables for two reasons. First, the speaker designer (Andy Payor) recommends Transparent Reference when the Aviors are reviewed. Also, I believe they are wired internally with Transparent. I was trading in Transparent Ultras and got full value on the trade, or it wouldn't have been affordable. (Full disclosure - when I listened to the Rockports, they were actually wired with Reference XL, but that was way out of my budget. I 'settled' for Reference.)
I really can't see ever going any further. As expensive as the Reference cables are, they are worth it (to me) for this level of speaker. But after the Reference level, Transparent's price curve gets really, really steep. And the Rockports may well be my last speaker purchase. Short of winning the lottery!