That does push us in the direction of speculation.
I'm thinking lean mixture.
Is the throttle response better when the engine is cold (ie, before the ECU is in closed loop mode)? Or on cold days vice warm days?
Apparently MG used a mixed plastic-metal fuel filter in some models. Is the V9 one of them? If so, a partial leakage before separation would reduce fuel pressure and affect atomization by the injectors which would lead to a lean burn.
Intake air leaks are another candidate.
Does the V9 have the V7 III's Secondary Air System? If so, problems including removal without an ECU remap would affect the AFR seen by the HEGOs.
There could be some electrical glitches but your description seems to point to a systemic fault which, like is said above, lean mixture.
That's all I got.