Hello  Thanks in advance for any help

I have a 2001 Sugar Sand Tango with 210 HP Mercruser it was running fine 45 to 50 mph with a 600 lb Load all of the suddent now it will not do over 15 to 20 mph at WOT. I took the Impeller housing off Coated with JB Weld Sanded down Replace extreamly tight Tollerance Impler looks Good I used Marine Grade Silcone around the ride plate to Haul gap trying to make sure I was not getting air bubles. I am at witts end I am a Disabled Vet very little Income. Realy cant even afford a Boat but I stole it for 500.00 running. But I cant give it up because its Just Good Family Time at least it was. Can anyone help I realy dont know what to try next. I have a very High Mecahnical Aptitude and acsess to allot of tools, I am just stummped at

1 What Cuased the Problem

2 How to Fix.

Thanks
GOD Bless
Marvin
321-369-8798