How much would you say these mods bring the sound isolation closer to the LCD-XCs?
I have never heard the LCD-XC, only the LCD-2, LCD-3 (both open) and EL-8C from Audeze so I can only comment with data.
From stock TH900 to the mods I described, a big difference. This is to be expected though. The TH900 is a dynamic headphone, just your "standard" 50mm moving coil dynamic, so no large planar magnetic diaphragm with waveguides and larger surface areas. The TH900, however, is a pretty
powerful dynamic headphone with a 1.5 tesla rated driver and can handle quite a bit of power, which make them really bass heavy and hard hitting.
It's essentially like a mini loudspeaker (more akin to a mini subwoofer) at this point, as speakers are almost always dynamic. So the mods -
thicker pads which attempt to create a tighter seal and more closed area between your ear and the driver to mitigate the leakage -- that's one part, however, the other two mods I mentioned:
dampening which works like/is literally acoustic foam that absorbs some of the resonant waves from the back of the driver as that's where a lot of the leakage happens (behind the driver - towards the casing), and
enclosure which is quite important as it would be on a loudspeaker; larger enclosure means that the wood can "absorb" a lot of the sound as it is essentially a closed seal (the one I use is about 2.5x the volume of the stock with much thicker wood, and of course dampening on the wood with acoustic foam instead of the stock cotton -- result is much more "room" and resistance for leakage vs. small/thin enclosure).
It's all wood based for the enclosure and the TH900, as mentioned, are very similar to a mini loudspeaker, so you can use the same basic strategies you would use when designing/modifying a traditional speaker for improving isolation on the TH900's.
Basically, the stock doesn't have enough of a enclosure (size, thickness) nor is it damped well (just some cotton by default) and then there's the pads which help to complete the front seal, though the enclosure & dampening makes a much bigger difference.
(Also, speakers are obviously "open" by default so you often hear people saying that open headphones are more like the sound of loudspeakers, which is generally true, since speakers don't have pads to create any type of seal. However, speakers are designed with enclosures of particular shape and size and materials, often some form of dampening inside the enclosure as well, to mitigate the soundwaves and various resonances generated as the driver moves back and forth)