1) about how many shots can you get off a 13ci tank?
150-160 rounds seems to be the average.
2) does it make sense to carry an extra tank? I've seen a loadout video where a guy carried an extra 13ci tank in a MOLLE pouch but unless you were playing a super long scenario game that seems like it would be overkill. I'd primarily just be playing Open Play at the nearby fields but I would also like to do a scenario Big Game whenever I get an opportunity.
In my opinion it makes more sense to get a larger tank, like a 45/4500. Otherwise you'll be having to trot to the fill station after nearly every game. At a scenario game this can be terribly inconvenient, especially when fill stations get backed up or go down.
4) I think I like this chest rig setup since it already comes with built-in mag pouches, but will it fit Dye DTM 20rd and PE CF20 mags?
Possibly, but it might be kind of tight. You're looking for something that can hold kalashnikov magazines. I recommend VISM chest rigs because they're super light, breathe really well, and are super cheap. https://www.amazon.com/dp/B005E98Z4K/ref=cm_sw_r_cp_apa_fabc_9TP8RK6RR0P6T2B5JKMD
6) I've seen a lot of videos of Magfed guys with scopes and red dot sights and whatnot, do they actually do anything or do they just look cool?
Most people don't like sights on paintball guns but I dig them, at least on my pumps and Magfed stuff. I'm fond of the cheaper holographic sights Westlake puts out. https://www.ebay.com/itm/301178838680
7) Where's the best place to get attachments for the rails and whatnot? Like foregips or lasers or camera mounts and whatnot. I think I'd like to put one of those angled foregips on along with a gopro mount.
I've had good luck with eBay. https://www.ebay.com/itm/402318179844
8) do barrel upgrades really matter?
Not really. The EMF already comes with a Shaft 4, which is an excellent barrel. The only reason to buy something else is if a.) You want something shorter, of b.) You get really into first strikes and want to try the rifled stuff.
Can you still shoot roundball out of FSR rifled barrels?
People say it works fine, but there's certainly no advantage.