Your serial number falls within the range of 280's that have a service bulletin written for the very problem you are experiencing. With all humility, ask your dealer if they logged in to the business portal and found the service bulletin and then followed the procedure EXACTLY the way it is written. Send me a PM if you want the service bulletin number.
To exotion: part of the procedure is cleaning the exhaust port/screen so if the dealer followed the procedure EXACTLY then that is a non-issue.
The problem is that some technicians will not take the time, especially on a new or newer machine, to remove a few screws/nuts/bolts to 100% verify everything. Echo did a great job providing a diagnostic procedure for engines that run poorly and if followed you will find the cause.
Here is a very important part of the procedure: For PAS-280 models you must have trimmer attachment 99944200540 installed for proper engine loading. If the dealer uses the wrong attachment they will not get satisfactory results. Just saying. String length is also a major factor. And the list goes on.
Get a detailed report of the work performed on your 280 and ask questions if it isn't written on the work order. Make sure you have the foam pre-filter on the outboard side of the main air filter. If the dealer simply replaced the carburetor without adjusting according to the procedure then that could be the source of the problem.