Almost 8 years dead on the thread necromancy! Congrats.
I think crits from a flaming burst weapon are intended to be pretty powerful. Plus, the warhammer is an x3 weapon, so should be more powerful on a crit.
Additionally, in reading the SRD, the 2d10 damage on the x3 crit is instead of 1d10 from an x2 crit. The d6 damage from regular flaming should always be applied.
So, flaming burst longsword on a crit is (d8x2) + d6 +d10, or 9+3.5+5.5=18 pts
Flaming burst warhammer on a crit is (d8x3)+d6+2d10, or 13.5+3.5+11=28 pts
And, my current party has a goliath barbarian wielding a goliath greathammer that does x4 on a crit. It also has the Impact special ability (the bludgeoning equiv of keen), so she crits on a 19-20. She has had some very memorable crits dealing well over 100 points of damage... (I did not add in STR & Magic bonuses, not to mention power attack, to the above)