Could be a number of problems but I would start with throwing the K&N away, its not needed and may be noiser. The standard filter is big enough.
As for the power it should be a revelation compared to the Serena, especially in the mid-range, but it is still a big car with 100hp to play with. What mileage is it ?
Does the Engine Management light come on ? Have you checked there is a bulb in the dashboard for it ?
If you have access to a scanning tool it may be worth checking for any codes which may be present. You may be able to clear them and then see what comes back before starting to replace things. And then start with the cheapest item before moving on.
I would also start with a quick check of the pipework under the bonnet - check for anything loose or worn, perhaps with a pinhole of some kind. Even a tiny hole will leak and affect how things run.
Limp home mode will be very obvious - it limits rpm rather than just holding back.
And also one other quick question - are the brakes binding at all ? Not meaning this to be funny, my mum had thousands spent on her company Vauxhall years ago (under warranty thankfully) because it was so slow only to find the handbrake never really disengaged properly - 2 minute adjustment and fix. And no she never noticed any burning smell, but she smokes like a chimney anyway
