Hratch,
We get about 6 knots at 2000 revs and yes we too are stern heavy. I also try to pack anchor locker with anything heavy, spare fuel etc.
I haven't touched the engine other than service stuff and nothing major to the boat reaaly. The fuel feed pipe from the tank was leaking and sucking in air. Blocked toilet is as serious as it gets, touch wood lol
Hope this helps mate,
Andy