The slow computer does of course explain the quality of backgrounds and models. I'm wishing you enough success so you can get a better rig and ramp up the quality of both. I don't necessarily think that the models look bad, just blurry and a bit uninspired. I'd also like more close-ups, the girls seem to be quite far away most of the time.
And why a spin-off? Just create a dream sequence where he is actually Sonic and gropes all the girls with his superfast hands. xD I'm all for unexpected crazy content. More Yasmine sounds good. I don't wanna sound mean but her model looked so much better than all the others. Whatever you did differently about her, maybe it'll help improve the overall graphic fidelity as well? If not, just give us more Yasmine.