http://i.imgur.com/JpwGur6.png
There is no point where Arcane (or Fire really) overtakes Frost, ever.
The reason we all were Frost for Highmaul is that the other two specs were still trash before the last round of hotfixes, and it really doesn't make sense to respec during progression unless it's absolutely necessary, not because Arcane or Fire are only viable with BRF gear. They are simply more situational and less versatile than Frost is.
Also, itemization isn't that important. You don't need x% of a stat for a spec to be viable; Fire gets massive crit passively, and Arcane is just strong on single target overall no matter the gear. What causes certain specs to shine is fight design, not gear. This is also why some people went Fire for Tectus progression, and Arcane for Butcher progression, despite having full multistrike gear, gems and enchants from highmaul.