I think that to fit the full system without also removing the standard Airbox would not be the best if you want to see maximum results.
With a Motohooligan Airbox and full Akra system I saw a healthy increase in maximum horsepower and torque when I had it mapped to suit. Can't actually remember figures but I think it was up to 75nms torque and 125bhp. ( I think the original power quotes for the 990 was 119 BHP)
I originally had Akra slip on silencers and when I fitted the full system the engine seemed to want to keep on revving much more and the peak power was slightly moved up higher in the revs but without a loss of mid range grunt.