It appears to be a common trait across a few of their boat models - you could even see it when watching the Big Angry Fish television program.

With regards to solving it - no personal experience, but I do recall one chap having success with installing a Permatrim - type attachment. 

A set of trim tabs, i expect, would also largely solve your trim problems.

Another approach - which should also help with the lack of 'get up and go' - would be to move to a 4 bladed S/S propeller. Broadly speaking these offer more efficiency - and 4 blades specifically, will have more blade area for initial push and typically more stern lift than your factory supplied propeller.

I once had a McLay 6 fisherman.

While i was out of the country my brother took it to a shop to get it serviced. 

The mechanic  insisted the motor was mounted to high and dropped it down two holes 

Totally killed the handling The boat no longer responded to trim and lost about 20% in top end speed 

50 kmh sounds very low for the boat and power what is your prop and  max revs?