When we got our 4x4 150 and it seems rather flat after the 170 we had before. We bought it with about 20k on the clock, it now has 40k. It felt slower than our 110 Fabia.  

   In the last few weeks it has felt more responsive. Is it my head or do ECUs learn your driving style? Can engines free off? Does a good thrashing clear out the emission system?  Thoughts?

Engines can free off but you would have thought that happened long ago.

The ECU will not have adapted. 

Weather changes but you must have had plenty temperature changes in the past 20,000 miles.

 

Is the oil due a change or has it been done recently?   It might be a bit diluted with fuel now if not changed recently, 

Has your Haldex had its oil change at 3 years / 30,000 miles. 

 

A thrashing is not a bad thing ( an itlian tune up) and maybe the tyre pressures a bit higher through the summer. 

 

I would hope a lighter petrol Fabia felt a bit nippier. 

If it drove 20000 miles on short journeys, the EGR valve may be almost clogged.

If you drive it now more "generously" form then, it may breathe a bit better and may seem a bit responsive now.  

I would think 20-odd BHP less and the 4x4 system sapping more power is the reason why. 

The throttle body may be clagged solid with EGR grunge.