I am surprised you did not have to pull the limiter caps. Any time there is a change in air or fuel, you typically need to make some adjustments.... The Echos run very lean already and then opening up the air flow is gonna make them even worse....
Mine was so lean it was crazy even with stock muffler.
Once muffler mod she needed re-tuned big time. Much better.
No cost muffler mod. Takes about 30mins.
Can cut that inner deflector out to use for like a gasket or toss it. Cut out to use it to keep screen in if wanted.