Yeah, you can remap the 35i and get even better results.
You can get a JB4 for the 35i which CAN give upto 80 BHP. Sounds a lot for a simple tune, but similar gain to the 335d, which can be mapped upto around the 360 BHP mark. While still being possible (easy in fact if you are relatively sensible) to hit the 40 mpg mark.
My average, over about 18k, in a remapped 335d was 37.8 mpg Compared to fuelly average of around : 37.2 average over 1,000,000 tracked miles for 2010 model, like mine). Could get 50 mpg on a really careful run, but I do a lot of fast motorway and town driving too, so 37.8 aint bad.
I suspect a tuned 335i wont be as good on fuel (Fuelly average for a 2007 model is 26.4 over 750,000 miles tracked), but then petrol is cheaper to buy (both car and fuel) so if you don't do many miles, you will likely get a better example of the 335i.
I think it was around 2008 for the updated nav, but don't pay too much for it. It can be retrofitted to a car, that doesn't even need to have had nav in the first place, for around £1100 if you have the ability to do it yourself.