Hi Jon.....welcome to the Forums
As Dave has mentioned, it could be a leaky injector....a simple leak-back test is all that is required to check this out.
Other possibilities could be that it is down to different driving styles, by the sound of it you are a progressive driver who uses the revs quite liberally....which may be different to how the previous owner drove it (e.g. low revs high gears) which would lead to carbon build up and of which you are clearing out of the engine and exhaust.
I wouldn't worry about it unduly at the moment and see how it goes once its had time to clear out.
Keep us informed on how it all goes
Cheers, Trevor