24/200 taken 3x per day is the commonly cited ratio and protocol for the E/C stack. This is what most of the studies followed and what's generally suggested, but higher doses have been tried with some evidence of higher efficacy--sides also ratchet up in a hurry, though, so the returns are somewhat diminishing.
Upping the caffeine component for more stimulatory effect is OK, but needs to be done in accordance with your own tolerance. I can personally tolerate about 600-700mg of caffeine acutely before I start experiencing undesirable side effects, but I'm a coffee-addled stim addict who probably needs an intervention; I know plenty of people for whom just the 200mg sends them into the stratosphere. Actual danger doesn't occur very high dosages indeed, as others have pointed out, but you don't need to ingest anywhere near 10g of caffeine to turn your day into a nightmare in a hurry. Reminds me of the evening I unwittingly downed ~1200mg of caffeine from canned energy drinks while pulling an all-nighter on a university project... I certainly stayed awake, but the shaky hands and jumpy thoughts didn't help with the work as much as I was hoping for!
