I feel it will affect the power output and fuel efficiency negatively as there is little outlet for the exhaust gases.
Check this, it is just like a choked silencer and if you replace it with the original ( as fitted by the company) you would find that the idling speed has increased, which reflects that with the modded silencer the engine was guzzling more fuel and needed higher idle speed setting.
Anyway, tinkering sometimes could have better results too.