I've got the full selection of front and rear sprocket sizes. In your case i'd definitely get a 40T rear and leave the front as the standard 15T.

16T just catches on a lug on the cases which i think is a mounting point for the ignition cover, though once its scraped a few mm of (unimportant) aluminium away it doesn't catch on anything else and never causes a noticeable problem during use even when you first put it on.

You can run a 16T with a 40T without having to worry about your chain's length. That's if you want to increase your gearing more for highway use.

Beyond that it all depends on whether your engine will pull even taller gearing - like with a 38T rear etc.

I found i still ran out of revs in 6th at a race track with a long straight with the 16/40 gearing. I've since bought a selection of smaller rear sprockets but haven't been back there on that same track layout to try them yet.
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