I thought I'd read somewhere that the Sport didn't come with the 'secret surprise' storage compartment under the passenger's seat, which I have seen on a Pop. I'd just accepted that, because the Sport seats were obviously a different shape

I had read the handbook and it just said "on some versions".

Well, imagine my joy today when I was removing the seat from the car (as part of a full-on sound deadening mission, inspired by pierce_jj's guide) and as I unplugged the airbag/OCS/seatbelt wiring, I thought 'that's a very large plastic box under there'.

After a bit of pulling and tugging, I figured out how to get it open! Happy day...

-Alex

I think it is speccing either leather seats or the Interscope hi-fi that loses you the under seat storage.

You don't get it if you have memory seats on the passenger side.

I don't speak or read italian so not entirely sure about this...

When playing on the FIAT.it configurator it appeared to me that you could specify leather and pay to keep the underseat storage (a box popped up showing the storage and a price in euros (about 50 IIRC)). Would be interested to know if this is the case.
While I'm on the subject of speccing leather on FIAT.it it appears you can have ivory leather seat tops with black leather seats, which looked rather swish!

John