I wish you all the best, really! Anyway I believe that it takes a lot of trust in oneselves gut feeling to do that but again, the money and the horn is yours and perhaps you will find an entirely new way to make a horn play better or worse, whichever way it goes. You probably like experimenting with things and who am I to prevent you doing that on your money..... .
However, at this point, I am curious of what is that you expect to find there where you are going, I mean, having a Mark VI which, I believe has no high F#, do you think that doing what you are going to do, will make a Mark VI of your III? If not, why? Doing it won't add anything to the sax world ( you have a VI and the cash buy another VI I take). If yes, do you think that what you want is perhaps the mechanics of the III and a sound as close as possible to a Mark VI?
Otherwise, beside the innovative thinking and proving everybody else's wrong, I don't see the point.
I am an iconoclast (would you believe it from my comments?) of the "underdog " persuasion so....I never went Selmer....until I could afford one... (so I am obviously jealous of somebody who has TWO and is about to wreck one...:twisted: ) anyway, I must admit that these beasts have something special. I just acquired a Mark VII (I told you about the underdog thing....) very affordable indeed. Can't think I would have the F# removed.
Look forward to see and (mainly) hear the difference (I would appreciate a full range of sound clips to show your horn sound capabilities before and after the "cure" so to speak)