You should be able to remove the injectors without actually removing the inlet plenum. RB20s you just remove the fuel rail. RB25s, it helps if you remove the throttle body to remove the fuel rail.
If the basic electrics are still in place, you can remove the CAS (leave plugged into the loom) and spin slowly by hand to cause the injector to pulse (with ign ON).
Better still, get the injectors to a cleaning service, they can evaluate spray patterns and clean the injectors. There are mobile services that do this. OK, it will cost you, but well worth it in the long run.