I have tried both equal length and different length XLR cables and I could not tell the difference. However, this was with a preamp with plenty of drive (low output impedance) into a fairly high impedance input. It's possible that a high capacitance cable with a source that has fairly high output impedance would have a more noticeable difference.
Keep in mind that in a studio environment, or live venue, balanced cables of wildly different lengths are used.
I have, however, noticed a difference with speaker cables of significantly different lengths and try to avoid this. I've found that shorter speaker cables are always better and given the choice of short speaker cables or short balanced interconnects, the former is the clear winner.
I have tried a number of different balanced cables (probably close to a dozen types). My favorite is Iconoclast 4x4 UP-OCC, but I've also had good luck with Mogami 2549 cable. The Mogami cable is much less expensive and nearly as good as the Iconoclast (in my system with my ears). Amazon sells a brand called Worlds Best Cables that uses this Mogami bulk cable with Neutrik connectors. I recently bought four 25ft cables for $140 total. Exceptional value.