Seems paying for seats with these low cost carriers is a rarity on here.
Depends on the low cost carrier.
EasyJet generally keep groups together, so I will only pay if I am travelling alone and it is a flight of at least one hour. Because they keep groups together it also minimises the risk of the middle seat: a couple will usually be given the middle and either the aisle or the window. When I've been given the middle seat I've been between two other solo travellers. EasyJet also won't let you change your seat once you've checked in, so there's no incentive for them to keep aisles/windows free for paying customers.
Ryanair will let you pay to change your seat after check-in, so they do have the incentive to keep the window and aisle seats free. They also don't keep groups together so that's why you're much more likely to be "randomly" allocated a middle seat.
I've never had more than 15-20 minutes in immigration, it's the same as before just with a stamp
Depends a lot on where you're going.
I've just got back from Ibiza, with only two desks was still through in under 15 minutes. And I was basically the 106th person to get off the Embraer 190. It seems to very much depend on the airport and how pedantic the passport control staff are; in most of Spain it's a brief glance and a stamp, but the Germans love to investigate everything, especially at Berlin.
I generally travel with hold luggage so the queue to get off the plane doesn't normally bother me. I can either stand in the immigration queue or I can stand by the baggage carousel waiting for my suitcase.