Depends what sort of MPG you are looking for ?
If you are used to a 1400 diesel you are going to be dissappointed!!
I average around 26mpg, closed loop will help, but you then have to figure in ANY cost of mods to achieve better MPG, as you don't make a return on the investment until it ihas paid for itself, so if it is only for ranother month or two, probably not worth doing anything other than making sure it is running right, and making sure the CTS is ok, ie if more than say 5/7 years old bung a new one in .
As said need more info on car, and what MPG you are getting now.
Coilpack will not do anything more than a group a coil if you are not having sparking issues but the group a coil is a massive benefit for the money, as are decent leads.
tabetha