sox original.mp3 newfile.changedBPM.mp3 tempo 1.025
This will give it an increase of 2.5% where 1.1 would give 10%. and the opposite is 0.9 would give -10%.
However if you want the pitch to change as well, just replace tempo with speed. Now instead of Mixxx dealing with the BPM change and slowing things down, just get the files as close as possible to each other and play DJ w/o the slow down.
I use a netbook so it's not that powerful and whenever I hit the "key" option to keep the same pitch and do a sync to beatmatch...all hell breaks loose and I get random sluggishness. So by changing the files beforehand, this should reduce the calculations needed by Mixxx. Haven't tried it yet as I'm at work but crossing fingers!