It's varied over the years.
Children of the Horned Rat 2E is the biggest advocate for Cathay actually. There are 3 mentions of Nippon in the book where they're talked about alongside Cathay. There are 22 mentions of Cathay in the book. That's 7 times as much Cathay as Nippon.
One part is about Lord Viskrin telling Clan Eshin to establish their stronghold in Cathay.
"Everything changed, though, with the return of Clan Eshin. Trained in the arts of assassination in Cathay, their black-clad murderers could infiltrate the most well defended lairs and slay the mightiest foes without ever being discovered." Page 31
"Assassin-adepts of Clan Eshin return from Cathay to serve the Council of Thirteen. Many Clans are quickly brought to heel by the assassination of their Warlords." Page 35
"Tilean explorer Marco Polare reaches Cathay, and writes of spying the Skaven under the great city of Wei-jin." Page 35
"Of all of Clan Eshin's warriors, though, the Adept Assassin is the most feared and reviled. These Skaven are masters in all of the techniques learned in distant Cathay and bring to bear an incredible array of fighting techniques that allow them to eclipse the greatest Human killers." Page 46
"Skaven smoke bombs are found among the Skaven of Clan Eshin. Using craftsmanship techniques learned in Cathay and Nippon, these small fragile grenades are filled with an explosive powder that detonates with a flash on impact." Page 74
"Learned from the Cathay sorcerers of the distant east, the Lore of Stealth is used by the ever rare Eshin Sorcerer. These spells are designed to augment the stealth, speed, and strength of the Clan's attack forces, and none are quite sure whether Clan Eshin Skaven's legendary skills have ever been completely mundane. Clan Eshin guards the secrets of this art to ensure that none of the rival Clans learn the answer." Page 78
"The mainstay warriors of Clan Eshin are the Night Runners. What differentiates these expendable foot soldiers from other Clanrats is that they receive rudimentary training in the fighting styles learned from distant Cathay" Page 96
"Only those Gutter Runners who excel in their trade can ever hope to be indoctrinated into the highest secrets of the Cathay fighting arts." Page 100
"The Art of the Silent Death wasn't the only thing Clan Eshin brought back when they returned from Cathay. A few Skaven delved into the black arts of magic. Blending what they already knew of the warp with the techniques used by Cathay sorcerers, they developed a new lore, one that serves to enhance their Clan's power and mystique. Eshin Sorcerers are mysterious, rare, and keep to themselves; they are well aware that the Grey Seers brook no competition from other Skaven spellcasters." Page 102
"Art of Silent Death: You have mastered the deadly art of open-hand fighting, as taught by the martial artists of Cathay. When making a successful unarmed attack, you deal Damage equal to SB–3 and Armour Points do not count as double." Page 103
That's more mentions of Cathay than mentions of Estalia in the main sourcebooks. I think the most interesting part is that Eshin Sorcerors learnt their Lore of Stealth from Cathayan Sorcerors.