Managed to get some semi-decent flying weather today so was able to fly the Magister with my new GP 3300 Nimh pack. It did take off from grass in the end but did take about 60 metres! Once up there was no massive improvement in performance as Joe said but the flight times are improved by a few minutes. However the plane did feel heavy with the new pack in it (weighs just over half a kilo!!!) and as soon as you take off the power the plane sinks, this did seem to make it good for landings though. I haven't tried any aerobatics yet as I'm only a novice pilot and haven't got the confidence yet - All my experience comes from simulators, and flying is a bit more nerve racking when it's for real!
I do really rate this plane and I think it must be really good fun to fly with a lighter and more powerful setup. Just can't afford all the upgrades at the moment!

Joe- Are you still using the Permax 600 brushed motor, even though you've upgraded to Lipos?