Yes, aftermarket barrels are legal in SSP, as long as same caliber/barrel length as factory, e.g. you cannot use 9mm conversion barrel in 40 caliber gun.
Too many variables... depending on your loads, your factory barrel, and most importantly your acceptable accuracy. For me, 3 inch or less 5-shot group at @ 25 yards with my loads is acceptable for IDPA/USPSA. YMMV