I have an MIM Strat that I want to put some SD Hotrails into, my question is: will 1 Hot Rail eliminate enough hum and make a good enough tone (I'm going for a metal tone, not deep chugging metal, more like twangy high-pitched lead metal) with the other two factory single coils?

Or do I buy the two ($150 for the pair, plus installation) for an awesome (hopefully) tone?

One more quick thing, would replacing the fender style bridge with a replacement from guitarfetish make a significant change in tone? I never use the tremolo so staying in tune is not a problem I'd just want a better bridge if it significantly enhances the tone...

"All matter is merely energy condensed into a slow vibration and we're all one consciousness experiencing ourselves subjectively, theres no such thing as death, life is just a dream and we're the imagination of ourselves."
If you're only using the Hot Rail the other pickups won't even be on, so no, if you don't use middle positions, it shouldn't make a difference concerning humming.

Unless I'm completley off-track
1 pup will do fine, the others won't effect the one in use
i got two dimarzio's and installed them myself a super distortion in the bridge(which would be perfect for what you need) and a pro track for awesome cleans and rhythm
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Yeah, if only the bridge pickup is selected (the bottom position on your strat), then you won't hear the other two pickups at all anyways. As for the other 4 positions, that will be a different story.

I am in the same boat as far as putting some SDs in an MIM strat. I was going to put Hot rails in the bridge, vintage in the mid, and cool in the neck, but I have decided on the Everything Axe set with the little 59er in the neck, duckbuckers in the mid, and jb jr in the bridge. The whole set is 205.95 if you would be interested. And the JB Jr would definitely be able to handle the sund you are looking for and I believe it sounds better clean than the hot rails.