OMC cooling problem?

[bigtony3ca]

I have a '91 Four Winns with a 454 omc king cobra outdrive. I think I have a cooling problem but not sure. When I cruise around boat works fine, operates around 150-160 degrees. When I push it the engine begins to heat up ( up to about 180-190) at which point I don't feel it is cooling properly and back off.

I have checked the impeller and it is fine.

Any ideas and also, how hot is too hot??

Thanks