First of all, the Sceptre/Tsunami single stern-drive models DO have the worst steering wander of any boat I've ever driven but one in 45 years of boat operation. (The 23' Martini small-block single Alpha I/O is worse)
Second, the absence of the torque tab will have a slight effect, but only slight.
Drive angle makes a difference, too. At displacement speeds, the deeper the drive, the better.
If you have power steering, check your actuator for play and your pump for fluid.
If manual steering, check the steering head.
4-blade props wander slightly less than 3-blades. (I don't know why)
While on plane, how much wheel turn do you have before the hull responds to the helm?
I used to have a 23' Tsunami w/ 225 Johnson, and it wandered so badly I would not leave the helm while under way.
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury
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