+1. Do it. Your fork is suitable and the distance you want to extend the threads is reasonable. BTW, ask the LBS if they can do it. If they say; "Sure, no problem, we've done it before" you should be ok. If they are hesitant, go elsewhere. You want a shop that's experienced doing this sort of work.

I once read; "If you ride a 52 cm frame, the world is yours." Apparently there are a lot of forks that were cut just a little too short for larger frames. Not everyone subscribes to the "measure twice, cut once" concept.