Rod 1
The DR400-160 is indeed a very good aircraft and probably the best of the DR400 bunch for the average club flyer, the extra 20 HP of the -180 results in a cruise speed only a few knots faster but it climbs better flys higher ( better TAS ) and lifts more payload and can have an extra 50 LT fuel tank fitted making it the DR400 to have if your mission profile is for a longer range.
In short there is no way to get more performance out of a fixed pitch four seat SEP. If you want more performance you have to start investing in CS props and retractable landing gear.