Actually, Adrenaline Boost still comes out as a good, cheaper version, provided you aren't intending on stacking up the Enhanced Reflexes tree. Enhanced Reflexes gives you an extra 1+1d6 Initiative (the boost to Reaction, and the extra die) for 1.5 power-points. 4 purchases of Adrenaline Boost gives you an automatic +8 Initiative, at the cost of 1 power-point and 4 points of Drain the turn after using it. Balances out pretty nicely, as it's a guaranteed boost, not a chance like an initiative die.
