Yeh - I don't think the ceiling dose theory is correct. What is true for me is that there's no point re-dosing until about 6hrs have elaspsed, as it won't add much. Don't know how psychosomatic that is though.
When doing DHC i tend to mix it with codeine anyway, but the effects are very different imo. DHC takes longer to come on and has a lot longer duration. DHC is also kind of stimulating but, paradoxically, more likely to cause me to nod.
Codeine is more mongy, comes on quicker and its euphoric effects are very brief for me - like only an hour or something (though I'll still be nicely chilled for a good few hours after that).
I would do DHC a lot more, but for the fact that it's only available OTC mixed with paracetamol, and I don't like doing CWE's with the stuff, especially as I'm a drinker so I'm worried about fucking up my liver. It's a lot easier/safer to extract from Ibuprofen pills, but you do have to pay a premium.