If Decating I would get it mapped (get an Apexi) at least you will get the full benefit of the mods fitted
and not run the risk of running lean..
The Apexi is a cracking ECU for very little folding these days, £350 odd normally 2nd hand, and if complete with the controller you can do all sorts of adjustments on the fly, + handy free gauges for Knock/Water temp/air intake temp/boost etc etc, believe you can even have pops and bangs put on the map at the flick of the rear heated window :P
Check with Scott or Simon on here but always thought that a more free flowing exhaust (decating) will make you run lean not rich.. as same fueling with more flow etc etc.. but I could be wrong.