I am not sure how many slots are available at STN particularly in the peak periods, but I would suggest that the average yield for AMS would be higher than STN.
On MV's point about EXT "syphoning" off BRS pass, it's always difficult to know what's going on unless you talk to the passengers. Many LCC pax are flying because of the low fares and either may have used an alternate airport before or wouldn't have travelled. You may be surprised how far people do travel to get a bargain - some people will effectively drive past other airports to get to the one with the lowest fares - or that they think have the lowest, if they haven't done their homework!