Higher ISO settings do more or less the same as what you are describing, in that ISO 400 uses the same exposure levels as with ISO 100 and -2 exposure compensation. The difference is that with ISO 400 setting, the needed +2 correction is done by applying four times as much analogue amplification to the sensor signal before A/D conversion, instead of doing it digitally afterwards. The noise levels are probably similar either way; there might be some additional in-camera noise filtering done at high ISO settings, but similar processing can be done later to the underxposed ISO 100, -2 output.
Maybe the earlier amplification reduces the accumulation of noise in later parts of the analogue part of the process, in which case using high ISO settings might give slightly better results than digital compensation?