Practiced Manifester won't do anything, since it only works up to your HD (however, if you were a telepath 3/rogue 2, for instance, your full normal manifester level would be 5, even though you'd only count as a 3rd level psion for powers known and such - 7 with Spellgifted and Overchannel for energy missile).
Even with those feats, spending 3pp will only do 3d6 damage (plus or minus, depending on the energy you use; those feats don't pay augmentation costs, after all, and you only get what you pay for).
However, you can go above your normal ML if you use Overchannel and Spellgifted, so you could spend up to 7pp to get 7d6 damage.