I would not let someone get around the redlining rules by doing it for less than a combat turn - that would, in my opinion, open the door to a lot of abuses, and not be in the spirit of that rule. I would read the rule more strictly and literally - you take damage for each combat turn in which you redline a limb; it doesn't state that the limb needs to be redlined for that entire combat round.
On the question of cyberlimb enhancements to customized cyberlimbs, pg. 44 states "Use the standard cyberlimb rules as described on p. 335, SR4, unless otherwise noted." The page number will no doubt be different for SR4A, but it is still obvious that customized cyberlimbs use the same rules for enhancements that standard ones do.