A dodgy lambda doesn't usually affect power - they tend to show up with a rough idle and a hesitation around 1600-1800rpm, which the cross-over point when the lambda heater circuit shuts off.
If your car idles okay and picks up smoothly then it has to be something else, not the lambda.
The MAF is usually the best place to start. If you have a Bosch MAF and it's new then it ought to be faultless. If you have a cheap/unbranded/unknown manufacturer MAF, then you gave someone money to take their plastic waste. Greta will be impressed..
but your car won't run right.
If the car has a manifold pressure (MAP) sensor on the inlet manifold, then for the money I would replace it, with the best quality part you can find. I'm not sure the MAP has as much influence as the MAF though, so there's no rush on that one.
I think it's a sensor/mapping issue now though, rather than a physical problem. If your fuel injectors were weedy or the fuel pressure was insufficient, the car would feel more jerky when it hit "the wall".. like it was running out of petrol. Yours (if I understood) goes okay but just doesn't have all its mojo?
Ralf S.