I run mine with the baffle in, it's too loud and annoying to me otherwise.
I don't think you will pick up any extra power from running without an insert either, unless you alter the fuel injection map at the same time. It's already lean with the stock can.
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'12 Husaberg FE570, 09 KTM XC-F/ 450 RFS hybrid, 07 KTM 450 SMR, 08+09 BMW F650GS twins/F800GS conversion, 03+05 YZF-R6