914 turbocharged power drop off at full rpm

Suggest you check the enrichment solenoid valve on the airbox - it’s a three way valve that actuated at 108% throttle ( but will also affect running at lower revs if it’s not functioning correctly). If the valve is sticking, or not moving normally it will affect power when on turbo boost.  

When you switch the master on you should just be able to hear or feel it click.  Still trying to get my head around the full affect of this valve, but worth a look. If it’s completely stuck, engine will not run off choke. Replacement is 6 to 8 week back order (in UK). 

You might look at your hoses between the turbocharger, intercooler, and intake plenum.  I had some holes in mine and the power would fluctuate at maximum.  I replaced the hoses and all's well now.