dont you mean, t28 with t25 rear housing. so it spools up quicker.
other way around wouldnt make sense. having a smaller front housing and bigger rear.
i was running a garrett gt28rs (disco spud) with .86 rear. for better breathing in higher revs. could have gone the .64 rear housing, would have better response but choked up in higher revs. ideally a smaller rear housing with an external waste gate was the better option. but getting the bigger rear was more cost affective.
that was on a ca18det.