Well since all the Gurus are probably not on yet, I'll give it a go,

;D

When you talk of neck tension as a number, .001, .002, etc, it's referring to the difference in diameter between the inside of the sized case neck and the diameter of the bullet you're seating. So, to your 270, the bullet nominally measures .277, and if you're sized neck diameter is now .273, then you have a neck tension of .004 . Changing neck tension can make a difference not only in accuracy, but in chamber pressure. If you increase neck tension, and change seating depth to say "jam" the bullet in the rifling, you can increase chamber pressure dramatically. Changing nothing else but increasing the neck tension will increase chamber pressures. This can lead to some loads in some rifles to achieve high pressure "early" in the load development.

The most notable for me was dinking around with the WSSM's. Neck tension was quite high, and pressures came on very early in development.

As far as lapping or polishing a bore, "it depends". I usually don't do it, unless I'm working with a problem child barrel. Normally it would be after I've exhausted all reasonable alternatives, and happens right before I pull the barrel and replace it.