it will need time to settle down as the fueling is a bit different now, it should start to burble and pop a bit more when warmed up, i find it well funny when i am coasting in neutral for about 20 seconds then rev her to 5k still in neutral and the popping is so cool when the revs come back down
the irmscher box is restrictive and if you want a few more bhp then put in a decent 2.5" cat back
the only time i heard an irmscher box sound decent is with a racing tube but even still u know it is all noise and no power 😛