I recognize that with spell haste not reducing the global cooldown, 1.5 second spells (scorch, flash of light) should technically cause "The spell is not ready yet", but with my 62 spell haste (1.44 scorch), i never encounter this despite using quartz /stopcasting to the best of my ability.

What's going on?

That's what's going on.
People with G15 keyboards and macros for it said that they could get their cast delay down to maybe 50 ms. Playing manually, my minimum delay is ~100-150 ms if I stop spells very early, and I tend to actually loose a spell here and there.

If you can really time 50 ms manually, you could cast 39 scorches in a minute, or 78 in two minutes? Try Dr. Boom, I know taht I can't squeeze that many casts at least.
It would take me 0.1s cast reduction before I'd start seeing "spell not ready" messages.

Edit: Heel mentioned 150 spell haste, that's ~7%, or 1.4s scorches.

You're pretty lucky. Since I got spell haste gear, I hit my quartz stopcasting macro like I am used to, and I get those 'spell is not ready' from time to time, which leads me to dps loss. This is mostly due to human reaction time, the time it takes for me to realize my next spell is not casting.

ie:
Let's say you have 2 pieces of spell haste gear, giving roughly 60 haste. You cast 1.44s scorches. If you stopcast at 1.48s, your spell will go through, but your next one will not cast (since you have not waited the 1.5s GCD server-side).

Personally, I believe the proper fix would be to have quartz modified so that it doesn't tell you to stopcast below the GCD.

Hmm... my latency has never been my strong point, i'd imagine that's it.

I wonder if blizzard's "jaw dropping mage dps" is with 0 latency, because my understanding is that continously timed casting classes (mages) constantly lose some dps because of it, vs. just sometimes for other classes

I have the GCD timer bar enabled for Quartz and use that for my stopcasting scorch spamming.