My ex-demo Plus 4 also had a 'flat spot' as you have described.
Not horrendous... but not a linear power delivery either.
A re-map would be in order... the GDi can be done to 180BHP from Williams Moran for example.
Got to be worth while doing!
I think the warmer weather with dry your gauges out as well!
It will all be worth it ">)
