Well I did the deed. Went what seemed to be the correct long coding route and it didn't take, so went on the app and spent 10 credits (could've sworn it was 50 credits last time I looked). Only been out to do the school run since, but pick up of 1st gear from a standstill without a huge amount to throttle seemed far more eager. Once moving, throttle response seemed sharper, not startlingly so.
Given the app only cost 10 credits, the method of long coding must be simple to implement if you know where you're looking.
Also activated the RS3 tail lights dance on unlocking - very snazzy.