A non-interference engine is only such as long as the distance between the piston crown and the valves isn't reduced beyond a given point. I don't remember how much room you've got on the 7M before that safety margin is gone.... But, that has absolutely nothing to do with timing.
No, even with slack being pulled up by the idler the timing is still skewed.... You're not removing the slack evenly, just on one side.
BT's right though, the difference is negligible and in most cases not a concern.
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