For a regular bolt-ons only LS the 4-2-1 is best. The LS motor isn't going to be able to take good advantage of the 4-1 unless you upgrade some of the internals with cams, higher compression, higher redline, etc.
Here's a pretty clean comparison on a Type R of the Mugen 4-2-1 vs. 4-1. You can see you really don't get good use of the 4-1 unless you can rev nice and high.
I suggest you read Mike Delaney's articles on header-exhaust design and its affect on power. It explains why you want the 4/2/1 and why you shouldn't look a a part for its individual whp gain, rather view it as a component of a system (h/c/e) that will yield much larger gains.
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