Depends on Hero/BL/TW usage in a fight. If at the beginning yes you'll get slightly better usage - but there's also the tradeoff that in BL/TW you cast faster and therefore use more mana. Longer burn means more Scorch weaving and therefore not the best possible use out of AP.
Does not matter if you cast faster during heroism, mana regen is also increased thanks to heroism being haste buff.
You're still going to burn through mana quicker with arcane power up.
Haste increases regen. But it contradicts itself when you have AP up.
Good luck maximising Blast usage during AP (which you should be doing) when you're burning through mana too fast and for too long. 15 seconds with BL/Hero is doable - if you're lucky with Missiles you'll stay ~90%, if you're unlucky you can dip to 80% but the increased regen from BL brings you back up when you resume normal rotation.
But at 7min 45sec unglyphed gets an extra 15seconds over glyphed, so it works both ways.
I think I'd simplified it to if the fight length was ~a multiple of 3 minutes +/- 1 minute, glyphed worked out better, if however it wasn't within this rule, unglyphed was better.
I agree that this rule is simplified; especially as up until the 7.45 mark that you outlined, glyhped will either equal uptime or be better, but I don't take it in cases where it's going to be "equal" uptime due to having to manage an increased burn compared to not having to do so. That, however, is more personal preference than anything.