Air flow could be an issue, water pan could be hindering temps. Might be inaccurate thermometer. How large are the holes in the doggy dish? I'd want lots of flow through that section.
Some people have reported that only drilling holes in bottom of pot doesn't allow enough air flow for easy higher temps. Far fewer complaints from those who cut out the entire bottom.
I'd ditch the water pan, or at least run it dry, just wrapped in foil. Are you putting in boiling water or cold water? That'll hinder the heat up process quite a bit.
Folks who build a WSM style fire ring out of expanded metal seem to have an easier time of temp control, and longer burns. This allows air to the coals on all sides, instead of having coals resting on the side of the smokey joe base.
Burning lump, or a combo of lump and briqs will get you higher temps with ease. Could be wet briqs too, that's always an issue if the briqs are moist.