what is too hot for a 454mpi engine. running 170-180 wide open, port engine running 160. New impeller, back flushed system. AM I being paranoid or is it ok

I would say you are ok as long as it is running good would not want to go much higher

warm lake water might have a lot to do with it also

before the impeller went out both engines always ran same temp at wot. I did replace impeller housing also.

Did the impeller come apart? If so did you recover all the pieces? They could be in the oil cooler, that would block the flow.

Mine always ran 175 with a 160 t-stat

The temps are not bad, but you might want to try a new T stat.
My 502mpi's run about 165 at cruise and 175 running hard.