Now, this is if the packets are hitting them at a bad time, ie: with the 100ms gap between syncs at its maximum, but still, won't syncing that infrequently result in very noticeable (and almost unplayable) delays?
Yes... Which means you'll need to do some magic to predict whats going to happen as well...e.g. rather than drawing the location where your oponent actually 'is', draw him where he will most likely be 60ms from now (intrapolate the movement directions and speed), give your bullets a wider collision/blast radius so you don't need a 100% accurate hit, etc.
You can also wait a moment processing a 'hit' until you verify with the other client that he didn't suddenly stop moving or stepped sideways in that past 60ms or so.
But either way, it's still a bit of voodoo to make a network game feel responsive when it's really can't be.