WoWProgress analyzed character profiles from guilds that killed the first boss in Tier 15 Jin'rokh the Breaker, and here is some info about thunderforged item percentage:

10-man
Guilds killed the first boss: 9616
Items equipped: 19123
Thunderforged items equipped: 2228
Thunderforged items percentage: 11.7%

25-man
Guilds killed the first boss: 1326
Items equipped: 7907
Thunderforged items equipped: 2033
Thunderforged items percentage: 25.7%

There are disenchanted, not equipped items among normal (level 522) loot, so we can assume that probably percentage of thunderforged items is:
- 10-man: 10%
- 25-man: 25%
which is quite significant difference.