If the above larger diameter headers and slip-ons were fitted to a standard unmodified SD (not R) what effect will it have. Will it increase/reduce torque/bhp, shift it around the rev range etc?

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.

The sound alone makes it a worthwhile mod Slip-ons sound good but the full system is simply amazing!!!

As Bic says you need to ditch the airbox to fully see the benefits of the full system. And get it mapped afterwards.

Thanks, very helpful information.