It seems like a 'total conversion' type mod, its the same city just in a different era, so very little is going to changed much, its just going to be slightly different models and new textures. Everything strikes me as being SR3 at the core, just modified cosmetically more than anything. Where you had an assault rifle, you have a space laser gun, the modern cars are now futuristic looking cars, and the missions, while new in story and directions, probably wont be anything which couldnt have been done as an SR3 DLC in the SR3 world.
I loved SR3, havent played the previous versions even though i bought SR2 afterwards, and ive been looking forward to SR4. I wasnt going to bother getting it just 10 days ago, instead wait for it to drop in price a chunk, but then found it online for £22 and figured i'd give it a go. It was a Brazilian site, but apparently it wasnt originally meant to be region locked and the retailer managed to get keys for each preorder customer in the region they were from, for their £22, rather than lumping them with a brazilian key or a refund.
I hadnt realised it was basically SR3.5 until a few days after though, which is a bit annoying, but at the same time i dont mind paying to get more SR3 and its stupid storyline. I would have preferred a true SR4 though.