1) A lot of people make the boxed-cake-to-cake-from-scratch (or frozen pizza or TV dinner or what-have-you) but I think that's really kind of an overstatement. Maybe more like buying some pizza dough instead of making your own, but you are still building your own beer on top of the extract - you choose the steeping grains, the hops & hop schedule, the yeast... So there is one extra element that you really gain control over with all-grain that can improve the quality of your beer. Namely, the mash. If you can do a good job maintaining the mash temp, and take control of your water (which may involve paying some attention to the Brew Science forum to some degree) then you can do well with AG. But a poorly done mash is not going to result in better beer than using extract. And that's really the only difference. You are making unhopped wort before the boil, and whether you do it with ground up barley or with LME/DME and steeping grains, the end result is still unhopped wort. It's still on you to get things right from that point forward. When it comes to people telling you AG is "better" than extract brewing, in the words of Flavor Flav: Don't believe the hype! It does not necessarily follow that AG will absolutely result in better beer than extract. Process matters.
2) If you do a one hour mash, as opposed to a 20-30 minute steep, then you are by definition adding at least 30 minutes to your brew day. You'll probably be using more water, meaning a longer time to get to the boil, so plan on adding at least an hour to your day. Then, if it's your first AG brew, double it, because you gotta learn stuff and that takes time. And add another hour buffer for "just in case." That may well be an hour of kicking back and relaxing with a brewski because you nailed your brew day, but hey... at that point, you've earned it
3) The answer: it depends. I mean, with practical certainty it can be said that there will be taste differences from one brew to the next even if you are just doing extract. And with AG, you are throwing some new variables into the mix. So, yes, a beer you used to brew with extract that you try to brew AG will almost certainly taste different. But really, the only difference is that you are controlling the wort. And, of course, the freshness - old, stale extract won't be an issue to worry about.
I like the beer I made with extract, and I like the beer I make AG. I think I make better beer now than I did when I used extract, but I don't think that's just because of the extract... I've learned, made mistakes, kept notes, honed in my process, and taken time to develop as a brewer. If I were to recreate an extract brew today from a recipe I used in the past, I am confident I would have better results than when I first brewed it.
If you are comfortable with your extract process and itching to try something new, try AG. It's not rocket science, and just like when you first started with extract, you'll probably worry more than you need to about whether you are messing it up or not. But you'll make beer, you'll learn, and you'll be better prepared for the next brew day.
On the other hand, if you are happy with you beer now and don't want to complicate brew day any further, don't add the extra stress. It's a hobby, and no matter what, you'll have better beer than if you went and grabbed a 30-pack of Molsen, eh?