Hi there- sorry to hear your troubles.
You can't run bigger injectors on the stock ECU as you have no way of informing the ECU that they are there. IOt will just assume they are stock.
Your R32 ECU can be chipped to run aftermarket parts quite simply. You need to get in touch with a stateside tuner and discuss things.
However, I would recommend going through the cheap and easy stuff before spending money on larger injectors.
Has the car been serviced recently? Fuel filter OK? When were the spark plugs last changed? Is the fuel pump working OK? If you get a long screwdriver press it onto the back of each injector and put your ear to the other end and check they are all working OK (don't do this in a tie or scarf and get your head ripped off by the belts!)
If the spluttering happens at a certain rev range, 4k to 5k, it might be coilpacks, but I would eliminate the others first if you have not already done so.
Cheers :cheers:
Chris