The published NPR is 1.5km either sie of the centreline of the NPR. The SID track and the centreline of the NPR are however not identical, as the radials change with magnetic varioation, whilst the centreline of the NPR is a DfT published line on the ground. The Stansted Flight Evaluation Unit (FEU based in Entreprise House is worth a visit to understand what is trying to be achieved on departure. Stansted are currently the only UK airport to fine operators £500 for flagrant off track departures. There are get out clauses, but please ensure you do all you can to remain within the swathes up to 3000/4000ft. Don't forget though at night (2330-0600) ATC have vectroing restrictions that mean they can not take an aircraft off the NPR until 4000ft.
Hope this clears up the track keeping issue.
To add to all that has been written, the removal of the speed restriction just take the max. 250kts below FL100 limitation off, if you want to decelerate, then there is nothing stopping you...