I find the TSMM line moves very quickly, so even if the sign says "40 minutes," you actually get on much faster (barring any kind of mechanical breakdowns). It probably depends on what time of year it is, but when we were there in December, the line for TSMM around 2:30 p.m. was way worse than it had been in the morning. Now I know there are other times where the line is supposedly better later in the day, but I am just reporting what we experienced when we were there. Now, we still got IN the long line at 2:30 p.m. because - as Bumbershoot said - it is one of the best rides ever and it is SO worth it to us - but we also did TSMM in the morning, right when DCA opened, and that got us ahead of tons of people. We did use the special GCH entrance into DCA and lined up by GRR with the CM escort, but you don't have to use that entrance into DCA to stand in that line. Sometimes, depending on the Cast Member, you won't start walking over to TSMM until 9:55 a.m., and sometimes the CM will start walking you over at 9:45 a.m. and you are on the ride right at 10:00 or before. Then, when you exit the ride, you see the enormous group of people in line (who you beat to the ride) and you realize why it was worth it to get on first thing in the morning!
So yes, if you go and stand in the line by GRR, you could look at it as, 'Well, if I am spending this time in line here, then why not just wait in the TSMM line later?" But there was a difference for me because we were escorted over to the TSMM line just before 10:00, so the time we made in getting to the line before the masses at the main entrance did was pivotal. And the wait by GRR may only be 15 minutes, or it could be 25 minutes, but it is still better than the wait time for TSMM during the peak crowds of the day. Plus, with the reports of crowds we have been getting on the DIS this week, I would not do anything to risk having to spend more time in line for TSMM when I could just book it over there first thing in the morning and get it out of the way. We don't worry about Soarin' until after that because it has a FP.