minor side note: flame turret is a summon. your enhancements don't boost it regardless.
a 6 level arti splash will still have terrible damage comparatively. a level 20 arti has a caster level of 20. a level 6 arti has a caster level of 6.
so, if you're dishing out 8-26 + 1-8 per level (that's 6d4+2, +1d8 per level if i'm not mistaken. but it might also be 2d10+6, +1d8 per level), your level 6 arti has a base damage of 16-90 damage, while the level 20 arti has 28-186 damage, before factoring in spell power etc.
with that being said, i've heard rumours that the rune arm may not specifically check your artificer spell level, but that it just checks if you're an artificer for use, then uses your highest spell level (or possibly highest arcane, only seen this mentioned with wizards). i have no idea if it's true or not, but if it is then you can get much better damage, though still only ~2/3 what a pure artificer might pull off.