1.8 TSi MPG Mapped/Tuned (potential leaky injector?)

[Bepis]

What MPG is everyone getting who has had their 1.8TSi GTi remapped/tuned?

For context, I have a manual with some basic bolt-ons and a stage 1 at 260bhp/450nm and suspect I may have a leaky injector.

Stock on the motorway I was averaging 39-42mpg indicated and no lower than 28mpg indicated when giving it some stick. A fill-up would usually last just over 300 miles with a calculated mpg typically of 36-39.

I'm fully aware since getting it mapped it will use more fuel when using the extra power but I now struggle to get over an indicated 30mpg on the motorway and I'm getting a calculated mpg typically of 22-26 and rarely higher than 210 miles from a fill-up. Even on a tank I purposely never did any hard acceleration and was mostly motorway driving I only managed a calculated 29mpg.

The other things leading me to believe I may have a leaky injector are:

After the car has been sat it will stumble and misfire for a couple of seconds then sort itself out typically only on the first start of the day.

The oil stinks of fuel and it's been in less than 5K miles.

The exhaust tips are very black and sooty

The exhaust in general smells very rich and didn't smell that rich when being dyno'd (still has the stock cat)

Other than these things the car drives fine, pulls cleanly and has no codes just horrendous fuel consumption.

Any ideas?