Red, what sort of mpg are you seeing on your 35i and how many miles are on it?
Let me first say the N20 is a great motor and the ZF 8HP is a great transmission. However when I check other companies mpg when switching to that transmission and using the same engine they did with their 6hps, the mpg increase isn't near where ZF and other manufacturers were, lets say expecting. Some 8HP marketing info I found mentioned specifically the 15% increase was in relation to the 1st gen 6hp. The 2nd gen 6hp itself was to have an average of 8% increase over the 1st gen 6HP, so only a 7% increase was expected at most from the 8hp over the 2nd gen 6HP. Other information I read seemed to indicate real world testing of the ZF 2nd gen 6hp and 8hp was really about half the % increase expected for various reasons including the fact that not all gears "increased" mpg the same (anywhere from 1% to 20 something% depending on the gear) and real world city driving didn't match as well to the computer simulated statistics they based the gear ratios on.
There aren't a whole lot of 35i owners and many haven't reported 5k+ mpg's, maybe it's time to bump that mpg thread and see what people are seeing and I'll avoid hijacking this thread.