Joel Freeborn has been experimenting with 6 column grids for iPhone and is sharing them with the Sketch community. Included grids:
• iPhone SE (15pt sides)
• iPhone 6/7 (15pt sides)
• iPhone 6/7 (30pt sides - as shown in screenshot)
• iPhone 6/7 Plus (27pt sides)
Check out Joes's portfolio and do let him know if you you have any suggestions for improvements.