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??
* Custom "does-it-all" Strat
*'12 James Tyler Studio Elite HD Burning Water (HSH)
* EVH Wolfgang Special Stealth HT
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.