I'm going to disagree with a few others here:
#1 More mAh does mean longer flight time. The more mAh (bigger battery) generally (but not always) means a heavier and larger physical battery.
#2 NiMh and NiCAD batteries can be interchanged in this and most instances. The supplied NiCAD batteries will be fine. It's true you will have to periodically cycle a NiCAD, but there's tons of information on the subject, and it's really not a big deal. I try to cycle once a month.
#3 The Superstar should have no problem getting 10min flight times, up to 15 minutes if you just cruise around. I have a buddy with one. I was impressed. It's not an overly fantastic plane, but it's capeable and nice, easy to repair and very simple.
#4 The charge time depends on the charger you use. A battery of this capacity can be charged in 15min, but in the interest of battery life, I wouldn't. Besides, if you get an extra battery and keep one on the charger, you'll need the downtime to let your nerves recover, especially if you're a bigginer. It'd also be nice if the motor/battery/ESC cooled between flights. A one hour charge time would be more reasonable. Charge the packs overnight (slow) before you go flying the next day.

Likely a LiPo cannot be used with the ESC (electronic speed control) supplied with the kit, because (as already stated) the voltage cutoff probably won't match..... unless it's a programmable ESC. LiPo's of this capacity are VERY expensive anyways. The only real advantage is weight. The plane was designed for the big NiCAD so I wouldn't worry about it.

IMHO