You can definitely get 400 bhp with the stock MAF and housing. I've done this myself.
Also there are two 10x1 MAF limit tables, one at CE3F and one at CE55. These allow you to raise the MAF limit - or you can just disable that nonsense by filling them with 255s
Im not talk about physical restriction, i was about voltage signal to ecu. I done 400bhp on stock maf also, but there is start to be lack of resolution for multiple boost levels or parthrottle accel at high rpm.
Two limiters tables, and others limiters i try to find not have influence on problem i noted, i think its must be somewhere some single value limiter of max kg/h value (MLMAX called in newer motronics) or something like that. I tested, last value on stock MAF curve is 1C95 hex at 5volt, which calculate is cca 914kg/h, i rescaled curve for 3.5" housing to read up to 1250kg/h at 5volt and every time maf signal passed 1C95 (which was in my curve little after 4volt) i recieved MAF error (incl.Injector light on dash). If you have different experience let us know.