Business class won't help you much, because TG uses the concourse terminal, and you need to wait for the next train, then get in along with everyone else who's arrived at the platform by that time. It'll only help you if you arrive at the platform just before a train is departing. But even then, your main concern isn't your fellow passengers on the TG flight, because a lot of them are Koreans, but whatever flights may have arrived just before your flight.

That time of the morning can be very bad for immigration. It's never bad for Koreans, but for foreigners you may wait up to 45 minutes. It all depends on what flights arrive just before you. If a flight arrives from Vietnam just prior to the TG flight, which happens a lot, expect a very long delay as they seem to take forever to carefully check each person flying under a Vietnamese passport. If the lines are fairly long, but with passengers just off the LH flight, the line goes much faster as they don't check out Germans/Europeans as much. It's real hit-and-miss. Sometimes I'm on that flight and I get to passport control and it's completely empty. Other times the queues are back to the wall. One huge tip though, look at the monitor sign boards above each booth. Pink is for foreigners, and I think blue for Koreans. Stand in the line of the last foriegner queue, with Korean queues on your right. If they finish processing all the Koreans, and nobody is waiting, they'll siphon off some people from the right-most foreigner line to the Korean booths. There's an immigration guy who does this and selects the people from that line to go over, so you cannot just go there on your own. But that right-most line will go significantly faster than any other line if this happens. It doesn't always happen though. If the flights just keep coming in, the Korean queues never empty and you don't get a chance to have that queue go quickly.

Having taken that TG flight lots and lots of times, your chances of getting the 7:10 bus are zero and very slim for the 7:35 if the queues are long. 8:00 should be fine unless you happen to get very unlucky or your flight is delayed.

I'm not sure how reasonable the 80-90 minute estimate for the bus ride is at that time of day, being it'll be the very peak of rush hour. I know the expressway is very busy during that time, but I've not timed it recently as I don't go that way anymore on arriving at ICN.

One word of warning if you take the earlier TG flight. It's almost always delayed. It's normal for it to leave about the same time as the later TG flight. In fact I've had on a number of occasions found it arrives in ICN after the later flight. The reason is that there's lots of Koreans on vacation to HKT, and other domestic locations in Thailand, and they catch the last possible flight to BKK, in order to maximize their vacation time. If the incoming flight is late, they end up trying to do some last minute duty free shopping, or just get lost, TG will hold the plane for them until they get there. Quite a few get mixed up and go to the wrong gate, thinking the later TG flight is their flight, not matching up the flight numbers, and have to go back to the proper gate after the realize their mistake. Anyways, it's very rare for that flight TG658 to leave on time, at least whenever I've taken it. TG656 (the later flight) leaves on-time more frequently.