400 to 150. That is more then 2 to 1 advantage, and as it was said, it is still growing. Also worth considering is that Allies are going to have less issues with fuel and spare parts availlability, for all they are a tertiary front, not to mention various nice things like radar or equipment to maintain and build more airfields. Truly, what were they thinking!?!
Also, if we assume this theatre is primarily IJAAF resposibility, that is 3 or 4 areas they will have to stretch their limited resources over. I mean, we have C-B-I and Solomons/New Guinea which are likely the main attrition areas, where Allied strength just keeps growing, not only in quantity, but quality as well. Then we have to add in China, as well, where losses might be substantially lower, but are still losses, which require more aircraft and pilots, not to mention requirements to defend Home Islands as well. Quality wise, their most numerous fighter was Ki-43, which will start to look rather lacking, as more and better Allied aircraft make an appearance. There were some decent aircraft around, Ki-44/61, rather comparable to Allied Aircraft of the period, but produced in rather low numbers, thus forcing the IJAAF to soldier on with Ki-43s.
Once Allies really start conducting large scale offensives, and IJAAF is forced to fight it out, it is really going to get bad. I mean, they might have 150 fighters, but there is always a certain amount that is kept in reserve/undergoing maintenance, further reducing the number of aircraft which will have to fight immense odds. When we add RN carrier forces raiding, and becoming rather willing to duke it out with land based aviation, as well as operational attrition and losses, it really becomes a downwards spiral. I would not be suprised if, by '44 or so, most likely cause of Allied aircraft losses is by AA fire, not enemy fighter activity.