Well, several things to consider-
1. If your armory is up to date, then you weren't optimally gemmed for fire.
2. Fire gets more out of raid buffs than Arcane does. Raid buffs include 5% crit, 5% crit debuff, 3% crit debuff, spell power from either a warlock or shaman. Fire receives more benefit to dps from all of those buffs than arcane does. This is intuitively grasped by knowing the spell coefficients for fire and arcane, and the stats table Xodiv linked should corroborate to this.
3. Your fight time from start to OOM was better for your CD usage. While your mana ended before your AP and Icy Veins ended, you still got about 9 seconds of uptime on them. That's 2.75 uses of Arcane power, and 1.5 uses of Icy Veins.
4. I know you said this, but you aren't used to fire in a raid.
5. Raid buffed, you'll get much more Hot Streaks than you would self-buffed. That plus more Ignites can result in a significantly different damage distribution among your spells. The increased Hot Streaks and Ignites would mean fire sees a bigger jump from raid buffs than arcane.
If you're doing normal mode ICC25, may I recomend the following bosses be done as fire?
-Saurfang if you don't have to dps the adds, Festergut to compare yourself to previous resets
-Professor Putricide (although this requires to be good about keeping up LB on PP while dpsing adds, as well as applying a single LB on the slimes assuming they live long enough to explode)
Three of those fights favor fire a decent amount over Arcane.
Yea, you won't need to scorch if you have a demo or aff lock, but do be aware that locks may or may not keep up the debuff below 35% because other spells naturally replace them.
Yes, potion of speed is still better than potion of wild magic.
All in all, I'd be sure to be able to get a parse for the bosses you're fire on and see how well you did fire, for a more accurate comparison to arcane.
Out of curiosity, what combat text mod is that?