I don't remember them (Sybas and Sabrents and Vantecs etc) being uncomfortably hot
So you actively measured?
Hopefully MOSFET will clarify that, but I assumed that by "uncomfortably hot" he meant "can you put your finger on it without it being painful". Which, assuming typical ambient temps, would probably imply a maximum rise above ambient somewhere in the 20s (C).
Some thoughts on your other questions:
If the card seems to be performant and stable with the stock drivers, I'd leave it at that. Definitely a case of "if it ain't broke..."
The temp differences with the case closed are not surprising. CPU has its own fan, so was not relying solely on convection with the case open. Closing the case probably resulted in increased turbulence of the airflow inside the case (providing more net airflow past the SATA chip's heatsink), but also an increase in the amount of warm CPU exhaust getting recycled around inside the case (given your lack of additional case fans). This would've helped the SATA card (since even a small amount of warm airflow is better than NO airflow, as long as the temperature of the warm air is still substantially below the temperature of the chip), but hurt the CPU (since it is breathing more of its own warm exhaust).