I resisted a full face for so long but I was so pleasantly surprised when I finally tried one and I slept better and had better numbers. For me personally, it seemed drastic to tie or glue my mouth shut when there was a mask specifically designed to eliminate the mouth leak.
I thought FFM was too bulky and scary looking, but it was fine when I tried it so I would encourage anyone with mouth leak issues to at least try it. Resmed often offers free return insurance if you don't like it after 30 days.
I wish I had tried the FFM long ago. I have a Resmed F10 but am going to try out the Simplus just because I really like Fisher & Paykel designs.
Good thread on mask leaks/air leaking from lips/ffm
Old82: Especially like your modified baseball cap to chin strap idea ... worth a try
Will try F&P Simplus and Forma on my on going search for better mask