As said BIN THE FSE, or sell it to some chav, with a 500bhp claim, sure to sell ha ha!!
Any alteration you make to either fuel or air will have a effect on the running, some more so than others, the chip will have been written for the rising rate of the std pressure eg no doubt, so with the fse you are going to have sjit loads more on boost fuelling even if the off boost/idle was correct.
You really do NEED to see what chip it is, before you go any further, if for example it is for a std engine then as you are on greys the fuel will be pouring in, as the grey injectors flow much much more than the std yellows, the point is you don't know so need to find out, before you buy anything else.
The ecu cover comes off by undoing 4 simple screws in each corner so not hard.
tabetha