I own both the Mk3's and X2's. And while I won't comment on the sound characteristics, as I haven't used my T50's in quite awhile.......X2 is immediately and IMMENSELY more comfortable. So much so, I have a hard time believing any person on the face of the earth would prefer the stock T50 comfort-wise.
It is possible to get used to the stock pads of the T50, but for me it took several weeks. I'm not familiar with the "X2-like mod" to the T50 that you refer to, but it should include a pad swap. I endured a fair amount of pain before I finally became used to the Mk3 stock pads.
I personally like Z Reviews quite a bit
Almost every headphone in my collection was purchased after watching his review, actually......and I'd have to say I agree with him roughly 85%. Small niggles here and there, but by and large his impressions match my own. My take on his videos? If you're new to the hobby and starting from the ground up, he nails it. If you've "been there, done that" and have a good amount of experience with top-of-the-line headphones, then you likely won't find much in his reviews to relate to.
I'm still new, haven't even been doing this a year yet.