How should I remedy bowed floor joists?
You'll end up getting different answers from different people.  Some folks actually will tell you to notch the bottom of the joist to let it sag back down before sistering the joist.  But IMHO, it would be best to know the cause and effect before making any permanent structural changes like this.  

I had a similar problem (high joists in poorly constructed house, not bowed) and ended up just raising the other joists to be level with the high ones.  No structural changes, just shims on top of low joists.  Resulting subfloor was dead flat when all was said and done.  If your joists are truly bowed up in the middle, it will be more work since you won't be able to use uniform thickness shims.  Shims on the outside will have to be thicker than in the middle....