My day to day development environment is on PHP 7. XF 1.5 is working great, but I would suggest 1.5.0 should be the minimum if you wish to use PHP 7.
Up to 70% faster compared to stock PHP 5.4+ but who uses PHP without xCache/APC?
Again, it doesn't make sense to risk the stability of your 1.5 site upgrading to PHP7 (you don't know if all your add-ons are compatible). It would be way better to wait for XenForo 2.0 and by that time there will be PHP 7.1 which will fix all the issues discovered in PHP 7.0 when put in production.