Don't reduce the shimming, it would only cause problems instead of solving any. Adding .1mm usually moves the power up, but it isn't guaranteed, plus you would lose some bottom end power as well.
A shorter manifold should give you more RPM, at the cost of losing some bottom end power. A different pipe could also make it better all round.
Carefully dremelling the exhaust port to make it open for longer can do the same thing as a shorter manifold, but this is a permanent modification and you could possibly ruin the engine if it isn't done correctly or carefully
You could also go a little leaner (if there is room for it) to get more engine speed, or you could change plugs to one slightly colder, but this would lose a little off the bottom end, and could also make it hard to idle nicely.

It's a lot of trade-off's to get more top-end, otherwise there's isn't a great lot to do.