Manual or auto no difference here, depends how hard you push the car and in fact an auto is more economical on the motorway because of the over drive and which gives 2500k RPM at 60mph.
Low boost would give bad mpg by the way. What mods does she have and why does she get a boost creep the air filter is too big?
The tyres are important for gOod mpg, properly inflated tyres give better mpg.
Possible sticky callipers - again bad mpg.
Many other reasons - even the grade of petrol she's probably using 95 ron which burns faster then 99.
For my auto glanza 500km or 300miles per tank is easily achivable on the motorway run but when combined at 0.9 bar boost I get at least 220 miles per tank and that's not driving like a granddad!