If it has a standard Mi16 engine and the conversion is done using the Mi16 ECU instead of keeping the standard 1.9 GTi one then you should get the same economy as any 405 Mi16 (or better as there is less weight to get shifted pulling away from lights etc.

But, you will rag it everywhere as it will be so much fun!!

P.S. About the Ecu's mentioned above, a 1.9 ecu might get just as good economy but I wouldn't know!