I wouldn't filter down the outside of cars - cars and trucks turning right
don't usually expect bikes to be there.
I think it's worth pointing out that filtering down the outside is much, much safer than filtering down the *inside* of traffic! Whether you choose to do it depends upon your confidence, the nature of the road and the traffic situation.
If I do it, and I often do, I filter down the outside carefully - greatly
reduced speed, so that I can stop before I get to any car which may suddenly move across my line without warning or any door which may suddenly be opened on me. Give as much space as possible, obviously, and look into wing mirrors to see what the driver's doing.
As for the discomfort angle, check your position on the bike. A lot of novices set the 'bars too high; this makes the bike hard to control and means that the impact all goes straight up your bottom.