the chip doesn't know how much fuel it is firing in, cos it doesn't know what injectors you've got. all it does it open the injector for a certain time, dependant on the engine speed and load (basically, there are other factors such as ACT, coolant temp...)

same as the load, it doesn't know what boost pressure you have as it doesn't know what MAP sensor you have. all it does is receive a signal between 0 and 5V and converts it to a digital number.

so a std 2 bar MAP sensor will give the full 5V output at 15 psi, but a 3 bar would only be giving 2/3 of 5V.

if using a 2bar MAP sensor with a stg3 chip, the ecu would get the full 5V signal and think it was at 2 bar boost. so it would open the injectors for a certain amount of time per revolution. as you are using std injectors, that amount of fuel would be less than if you were using the proper 803's for the stg3 chip, by approximately the same proportion as the flow rate ratio of the injectors.

the fuelling will probably not be far off, but i wouldn't want to do it. besides that, the stg3 chip will have a different ignition curve in it, as more boost normally means less advance, so it wouldn't perform very well on full boost. the transient response, and low load stuff would probably also be awful.

no need to do it. a decent chip is just 200 quid from motorsport developments, so do it properly.