When using your demo as is with current Xcode 7.3 and e.g. iOS Simulator iPad Air 2 on iOS 8.1 (12B411), the margins are wrong compared to the separator insets:

After trying this I used the nib instead and made the following adjustments (as you had hardcoded +/- 15 points as margins), which should work:

CellIdentifier: Preserve Superview margins checked
Content View: Preserve Superview margins checked
Leading/Trailing Constraints: Each leading/trailing set to constant 0, but pinned to superview's margin instead of superview

Expected:
Margins automatically adjusted

Actual:

I am trying this for about a week (!), do you know of any official way to get the margins right with iOS 8.1 and 8.2? Btw: Margins are broken when used in Form Sheets with iPhone 6+ in Landscape with iOS 9.x, too (they differ from the separator insets as well).

Any idea?