I just got mine and have not tried to neck size only with it.
To answer two of your questions:
1) using this press will not affect neck tension. Neck tension is a function of the size of you die busing and/or sizing ball (if you use one). The way the press works is you replace your die lock ring with a Forster ring which is sized to fit into the slot in the die frame. This allows the die to "float" a little, but that float won't affect neck tension, per se. The OD of the neck will be the same size as what your current die/press throws off. But it may be more concentric, due to the float.
2) I think the Redding competition dies have an internal sleeve which centers the case in the die so that the bullet is better aligned with the neck while seating (as does the Forster seating die). This will be unnaffected by use of this press, once you get it adjusted. and reset the lock ring. You will find that you'll have to recalibrate the micrometer top if you record settings for different seating depth/bullet combinations for this press - but you'd have to do that if you changed to any other press than the one you have.
I'm getting measurably better runout with the Forster press than with my RCBS. So far all I've seen is upside with no downside. Maybe others will comment.
Elkbane
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