It’s not the Sport model, it’s the 12MY with Sport Pack.
My understanding is that MY12 GranCabrio Sport was made available from Sept 2011, aligned to the UK Maserati press release, the base model was classified auto.
I'm not intimately familiar with the Imperials GC, it looks like a sport with the optional MC sport line kit, see below the Sport Line accessories weren't available on the base GranCab.
The MY12 sport model updates listed in the newspress UK Maserati release , scroll down for detail:
http://www.maserati-alfieri.co.uk/alfieri06qqqqqqqqq.htm
So what are the mechanical differences, if any?
Assuming you are referring to the MY13 changes, these inc. front face-lift bumper & lower spoiler, deleted front fogs, tinted rear lights, seat insert & front seat surround, steering wheel, LED running lights, headlamp logo, engine output increase of 10hp, with additional trim and body colours available.
I believe the same mechanical spec continued to the 'run' out model MY18, apart from subtle interior changes, SatNaV & control, sound system and front and rear bumper updates!
Regards.