i dunno how wrong this tool is but
http://www.wowhuntertools.com/hunter/
u just click upload from armoy, it takes all ur gear in then u click test or update and it gives u a dps score. one of the tabs will take u to the options and u can choose all the options there like what food u are buffed on, what scope u are using, if u want to include talents how many targets wat gems what enchants u r using. even if its not up to date with ilvls and stuff i imagine if everything is still a controlled variable it will give u an idea of change.
so if it shows 40k dps with mastery and u change it to multistrike and it then drops to 35k when u reload it, fair to say its a big decrease regardless of true values as long as weights r correct.