Change the fuel filter, seen this many times good response on boost terrible off boost, had it on mine when I got it nothing at all below 2500rpm, changed fuel filter and it pulled nicely off boost.
I would service anything that you can, misfires are usually caused by small dodgy bits, such as leads/plugs.
Unless it's new put in a new CTS as a minimum, but do airtemp sensor as well would be better, but defo CTS(coolant temp sensor), this is what determined the vast majority of the air fuel ratio.
A stage 1 chip brings a massive increase in low down off boost compared with a std set up.
It would be a good idea first to rewire the FAN/s and the fuel pump, these are notoriously crap on cossies.
At stage 1 I would use AGPR12PP8 plugs from ford, but others have thier own favourites like 071(std rst plug).
Don't take for granted that a coilpack conversion is going to cure a misfire, it won't, unless the misfire is caused by the coil, the original problem needs sorting.
A looked after cossie is capable of very high bhp before misfiring becomes a problem due to boost, a lot opt for the "group a" coil at around £35.
But once you have done all the basic stuff to save you money, a check is well advisable.
tabetha