By 'injector' you mean main jet, yes? And bear in mind that if you remove restrictions to the inlet tract too, the jet needs to go up another size (or so - it's all trial and error really).
I had straight-through pipes on the Bonneville and took some restrictors out of the airbox. I had to go up to (from memory) 130s from the standard 118s. I'd estimate I got another 5-6 bhp from that. But try raising the needle first, as that costs nothing more than a couple of hours fiddling.
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