you'll proably find your air filter if its an open cone is suffering heat soak, and will loose you some ponies.
also a stainless exhaust may loose you some torque as well.
mods only make a difference once the car has been remapped to make full use of them.
go have a search for abbey motorsport and rs tuning, there are lots of dyno grpahs in them, especially the abbey one. showing pre and post tunes. also shows what mods people are using.
as for sparks, have a look on the envy website, under their service section, you can pick them out in their and it will give you a price as well.
sparks on the zed are done every 52,000 miles which is a long change interval, if your near that they won't be as clean and sparking as well as they would at 20,000 miles.
also dynos are only good for benchmarking.
so if your planning to mod it tells you your starting point, and you can then see what your improvements are from there.
don't base your figures on the stated manual ones, you may never have had all 276bhp to start with, others have had more.