2007 Mercruiser MPI 5.0 engine not making power

OK Guys, I feel like I have exhausted all paths of possible issues with this engine, so I turn to you all for something I may have missed. I'll try to list everything that has been done but it was at the mechanic shop recently so some things may have been done I am not aware of.

Let me tell you about the issue I am having. It all started a couple of seasons ago and was mostly intermittent. the problem started showing itself in the form of 1. not wanting to crank without applying a bit of throttle to get her going and once going for approx 30 seconds, I can back down to idle and she just purrs. That happens every time I am starting the engine cold, once warm, it will crank right up. 2nd issue is when increasing throttle to get on plane. I noticed that I would have to add more throttle than normal and also would need to shift weight up front to help get the nose down. Once on plane, the throttle would need to be slightly increased more than normal and when monitoring the GPH, It would require about 1-2 more GPH at 4000 rpm than normal. It felt like maybe 1 cylinder was not producing what it should be. That went on throughout last season. I could always get on plain, eventually.
NOW, this season, I put the boat in the water and same thing with cranking, had to add throttle but when I go to get on plain, I can burry the throttle but can not get above 3000 rpm's total loss of power. seems to re running with 2 cylinders down and rough feeling. I also felt something knock in the engine a couple of times when I had the throttle all the down and at approx 3000 rpm's. I backed down the throttle as soon as I heard/relt the knocking. NO beeps from the smart craft and no codes on the gauge.

These issues still persists even after the following being done. Replaced IAC with Mercruiser brand valve, replaced the little filter leading to the IAC. cleaned the air filter. Tested for fuel pressure, it was good but decided to replace both fuel pressure regulators and both fuel pumps with walbro pumps in the cool fuel housing, I cleaned the pickup screens while in there. I have replaced the spark plugs, wires and the distributor cap and rotor. The mechanic said he checked for timing at the distributor and says everything is tight and spot on when the piston is TDC.I checked for spark using a timing light and all plugs seem to be firing consistently. I checked pressures in the cylinders and they range between 130 and 160. Mechanic says there should not be more than a 10% difference between the readings but I really don't think that would be the cause of my issue. I drained the fuel tank and cleaned it out the best I could and the fuel is clean. I change the fuel filter a couple of times a season. there is trash and some water in there each time I go to change it though. Last but not least, I had the mechanic clean the injectors and they said they were not all that bad but was able to get a better flow after the cleaning. 1 last thing to mention, when this issue began about a year ago, I replaced the plugs and the plugs from cylinders 1 and 8 had the ground electrode burned completely off. All other plugs looked pretty good.
I am at a loss at this point. Mechanic says the next steps are to pull the heads and inspect the cylinders and replace/repair as needed. I really don't want to go that rout if I can help it.
Any help is appreciated. Thank You for reading.

 

Oil looks fine when changed. They changed it when it was in the shop and didn’t mention anything. Engine hrs are approx 550.
Fuel pressure was tested on the trailer. Not under load. New fuel pumps, and pressure regulators. I cleaned the cool fuel module and didn’t notice any paint chips in it.
All injectors cleaned and are now in spec.

 

There is a filter inside the cool fuel module and it was spotless. Fuel pressure was about 50 psi. Can’t remember exactly though. Mechanic said it was in spec. This is a mercruiser certified mechanic. Fuel rails were cleaned when injectors were cleaned.

 

OK, Update!. I ran the boat this weekend and was surprised. The starting issue is unchanged, I had to give throttle in order to start the engine. once started, backed the throttle down and she idled just fine. I idled and drove at apron 1k rpm through a no wake for about 6-7 min and when I hit the normal operating speed, I slowly brought the rpm's up to about 2.5-3K and she sounded smooth. I began to push the trim tabs down further than normal, the bow dipped and I began to feel her picking up speed. So I began to lightly apply additional throttle but kept it aprox 3.5k. She ended up getting up on plain, engine felt smooth and 3.5-3.7k rpm continued to feel smooth. I did not try to go full throttle because things were going so well at the time. I did this going to my destination about a 15 min ride there and a 15 min ride back. both trips were the same. everything felt good. I'll continue to put her through more paces as we have more trips and report back here with anything interesting that happens. I wonder if there was just a touch of bad gas in the lines that was messing with the test ride after the work that was recently completed.
Thanks for all the replies so far.