Your component values look okay for that frequency.
You might want to drop the operating frequency. (The reference circuit you mentioned uses 44kHz). Operating at 300kHz will increase your switching losses.
You may want to increase your capacitor size to reduce the ripple, although it's probably not a problem since it's going to a battery.
Is that diode drop from one or two diodes? Are you using schottky diodes to minimize the diode power loss?
The 2.6V drop for the transistor is much too high. The BUZ11 has a maximum on resistance of 0.04 ohms with 10V Vgs, so it's drop should be much lower than that.