Taking on some of what Emperorswrath has to say, and then just quickly spewing ideas onto a page.
What if the Psker level limited the range of powers a psyker could buy?
E.g.
Delta: Can only buy Minor Powers,
Beta: Can buy up to two disciplines, or master just one.
Alpha: Can master upto two disciplines
Alpha plus: Can master two disiplines and buy a third, or master one discipline and buy three others.
or maybe just even limiting it to particualar target numbers?
Maybe rather than learning Psy level, players could choose to buy additional powers or more psy dice. If the cost on Psy dice was particularly high you'd end up with Delta's with all the Minor powers able to maintain several at once (due to having lots cheap powers and a few dice), and Alpha Plus's either with a few select abilitys and suffcient dice or a broad range and no power....
If normal Psy dice had a much lower chance of invoking the warp the GM could then tempt power starved players with the offer of lending them some extra 'warp' dice, which could be rolled even after a failed check. These dice could have a much higher chance of peril or automatic corruption/insanity points etc.