The cans will do little but sound a bit better.
The MMax box is rubbish, it's just something else to go wrong if you ask me, put it in the bin and do the wiresplices instead, detailed here.
A couple of 1 1/2" holes in the top of the lid (one either side) works as well as anything else.
60+bhp is available on a '21 with no other mods, so 30% over std.
Yes, you will need to rejet.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread