We had a similar issue with a Mariner after a line degraded and filled the filter with the hose linings.
Replaced the hoses and filter and it acted like it was lacking in fuel again at high rpms.
After scratching our head for a few weeks checking the lines and filters, we replaced the Marpac filter with a Sierra filter and the problem went away.
We cut open the Marpac filter and found nothing wrong. Only thing we could figure was the composition of the filter severely restricted the fuel flow.
The Marpac was a 10 micron and the Sierra was a 21 micron.
If the transducer is performing well at the speeds you want it to, just make a plate out of plastic cutting board material and place over it or better to mount to. Something similar to this.
