It is high time that pool/billiards has similar player rating like ELO rating for chess. In chess, as soon as you start playing your first tourney, you get an ELO rating which is calculated for your every win loss or draw in tourneys
If there is such a rating then it will be easy to measure the strength of a tourney field by using the average rating and it will be probably the most accurate measure of strength of field
Sadly we do not have such a rating one way may be to use the WPA rankings and find how number of players with WPA ranking points playing in All Japan vs the number  of players in US Open. Maybe our resident Nate Silver, AtLarge can help us. Again there may be shortcomings as there are good players not playing in WPA events etc
Anyway I use a simple method-going thru the lists of players for both US Open and All Japan for notable players , 1st tier and 2nd tier world class players
Of course this is subjective method and I may miss out number of names especially most of the Japanese players who I have never heard or -maybe someone more familiar with Japanese players can give an idea. I do agree with you that of the 26 Japanese players in the last 64, most do not have much chance of getting into semis - I based this assumption on fact that Japanese players on won All Japan twice in past 10 to 20 years and both these winners were W9B champs. No offence to Japanese players but I am not knocking them just trying to state the stats 
Anyway the total number of notable players in both events is around 30 to 40 which says that the strength of field at top is about same. 
Again that is just my guesstimate opinion :grin-square:
US Open
 Shane Van Boening - USA
 Earl Strickland - USA
 Efren Reyes - Philippines
 Johnny Archer - USA
 Francisco Bustamante - Philipines
 Mika Immonen - Finland
 Darren Appleton - England
 Jose Parica - Philippines
 Dennis Orcullo - Philippines
 Rodney Morris – USA
 Niels Feijen – Netherlands
 Thorsten Hohmann - Germany
 Corey Deuel - USA
 Ralf Souquet - Germany
 Nick Van Den Berg - Netherlands
 Alex Pagulayan - Canada
 Warren Kiamco - Philippines
 Nikos Economopoulos - Greece
 Justin Bergman - USA
 Justin Hall - USA
 Oscar Dominguez - USA
 Jeremey Sossei - USA
 Mike Dechaine – USA
 Jayson Shaw – Scotland
 Jason Klatt – Canada
 John Morra - Canada
 John Schmidt - USA
 Karl Boyes - England
 Albin Ouschan – Austria
 Stapanov Kontantin – Russia
Waleed Majid
Dennis Grabe
Radoslaw Babica
Imran Majid
Shawn Puttnam
Max Eberle
All Japan
Jeff de Luna
Rodney Morris
Waleed Majid
Ko Ping Chung
Carlo Biado
Takahashi Kunihiko
Ramil Gallego
Warren Kiamco
Li Hewen
Dennis Orcullo
Thorsten
Lo Li Wen
Johann Chua
Radoslaw Babica
Ko Pin Yi
Mika
Antonio Lining
Oi Naoyuki
Ralf
Chang Jung Lin
Dang Jinhu
Johnny Archer
Liu Haitao
Chang Yu Lung
Daryl Peach
Fu Che Wei
Wu Jiaqing
Akagariyama Yukio
Cheng Yu Hsuan
Jeff Ignacio
Aloysius Yap
Raj Hundal
Wang Can
Lee Van