1. Whether people notice differences is not the criterion. After all, they're more likely to notice changes for the worse. What matters is the overall band sound.
2. For guitar FX, it is very hard (impossible?) to beat a high-end multi-fx unit. Example: Dweezil Zappa. Ideally through full-range flat-response amplification, though of course it can still sound great through guitar amplification. (Also if you are modeling a classic amp/cab and put the signal through a [i]guitar[/i] amp, then obviously the modeling won't work, however pleasing the result may be.) And you'd probably need a midi foot controller. So we are talking quite a lot of cash, but probably cheaper than having twelve individual best-of-breed pedals. Many guitarists (conservative bunch) are too set in their ways (or too poor) to take Dweezil's approach.
3. Whether it's multiple pedals or multi-fx, you've got plenty homework to do figuring out how to use the gear.