There is just no comparison between Deans and Tamiya; the Deans are simply a must. They are far more reliable, and yes you will notice a difference in performance in both duration, and power. The deans are basically far more efficient, and let more amps go through them, giving your motor the sufficient amount of power rather than being limited by Tamiya connectors.

For top-notch battery packs you wanna look for either IB4200's, or DTX 3800/4200's. I would reccomend the DTX 3800 SxS, but they are hard to fit into most stock models. I had to dremel my bandit to get them to fit in there. They provide far more punch than normal IB4200's. However IB4200 WC are just as good, but very expensive.

XL-5 is limited to handling a 15t (540 size) and a 12t (550 size). you already have a 12t titan (550 size motor) in your bandit, making it the most powerful setting with your current ESC. Try high gearing for speed, low gearing for torque. If you are not satisfied with your system, try using a 7-cell, or 8-cell battery pack. The XL-5 can run 4-8 cells (4.8v - 9.6v)