I haven't used it with a tripod as of yet I hand hold as I walk around the reserve I don't find it a problem, however I'm not holding it to my eye for long periods only when I'm taking the shots.
I know its recommended that if using a bigma on the A700 you set the focusing speed to slow to reduce the chance of gear stripping as the motor on the A700 is known to be quite torquey so maybe that is the reason its slower on the A550 (less torque). Of course that could be another reason to get the 150 or a brand new bigma the gears won't strip because it uses the lens hypersonic motor and not the camera's AF motor.
The 150 is slower on the A700 than on the A550
If I was looking at buying one today I think that I would get a 150 or brand new bigma to go with one of the newer Sony models (230,290,380,390,450,500 & 550) for the older models (100,200,300,350 &700) I would look at getting a used older style bigma.
Of course then there's the Tamron 200-500 I haven't used one yet but I read plenty of reviews stating that its a great lens and is equal if not better than the bigma.
Derek