#1
so I've heard from some that if i change the stock, thin zinc tremolo block to something like higher-mass brass or steel block, it noticeably improves the tone and the sustain.

Likewise, I heard that changing the saddles to steel saddles or those weird TUSQ stuff improves the tone/sustain as well.

I guess switching out the block isn't too much trouble, and it shouldn't be that costly (as it's about 30 USD from guitarfetish.com)

but does it really improve the tone noticeably??
Guitars:
* Custom "does-it-all" Strat
*'12 James Tyler Studio Elite HD Burning Water (HSH)
* EVH Wolfgang Special Stealth HT
#2
The key word is 'change'...not 'improve'. I have a dozen Strats and favor the stock bridge (a few I've turned into hard tails).

There's no harm in experimenting but keep your original parts. Even so-called H-igrading can devalue the value of a Strat or Gibson. Most folks want a known variable when buying a guitar and notwhat might be a iffy custom job.