First off, you will never
get an automag pistol to be as efficient as other pistols, like a TPX, so if efficiency is really important to you, consider getting something else.
But, if you're determined to use an automag, there are some improvements you can make. If you use a barrel kit and have a good paint/barrel match (which improves consistency), you should be able to get more like 15 decent shots off.
You could also get a bit more drastic and mod the valve itself (like ScreamingShiek mentioned). This may improve the efficiency another 10-20%. Smoothice on AO did some interesting tests on some different things he tried. Automags.Org Online Forums - Valve Modification Efficiency Testing