Mercruiser 5.0 overheats

I have a 2005 Maxum with a Merc 5.0L efi motor, which has been running hot. I replaced the impeller and thermostat myself, then ran out of ideas. Took it to the mechanic, and they said I need new manifolds. The boat is 6 hears old, and the engine has 300 hours on it; almost all in salt water. I store the boat dry, and flush the motor after each use. After market manifolds and parts would save me about $600 on the bill. Any comments on the use of after market parts would be appreciated...

Any comments on the use of after market parts would be appreciated...

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Osco and Barr manifolds are of good quality. GLM's quality is questionable.

When you replaced the pump impeller, were there any missing vanes?
Is so, they are likely lodged in the fresh water supply system in the transom hose, PS cooler, or T'stat housing.
They need to be removed or you will be no further ahead.
Otherwise heed Guy's comments.
You can pull your manifolds and risers for inspection first before you rush out and arbitrarily replace them. Though six years in salt water service sounds about right.
There is another current thread dealing with the merits of boat pulling and flushing versus leaving it in the water. Good food for thought.

Rod