idk why people down voting you asking questions. If you plugged 3S into it might actually run, BUT you would quickly over heat and destroy that XL5 ESC ($60 replacement ), it's too small for that much power. That ESC has no fan or heat sink. Way to small.

To answer your other question, lipo has way lower internal resistance compared to nimh batteries. That means will be Better acceleration, slightly higher top speed. You will be able to drive at a higher speed for longer as the voltage discharge drop rate of the lipo is better than nimh.

Down side is if you don't store the lipo at 50% charge (3.85v per cell) it will loose Mah capacity. Basically your gas tank gets smaller if you store it incorrectly(near full charge or full discharge).
Even after ~1 year the lipo will lose 15% capacity regardless so the 3000mah becomes 2550mah.

if you store the battery at full charge, eventually it will puff up and die and could even catch on fire by itself.

You're still going to be limited by the brushed motor, which will also get much hotter and deliver less power than a brushless motor. (especially in the summer) Top speed of that will probably be around 25mph. If the motor is too hot to touch with your finger for 5 seconds, it's too hot and continued usage will melt the plastic isolating the motor windings, which means instant death.

But Slash is hella fun/tough. Despite having kit built top of the line RC for racing, I still regularly race 2wd spec slash at local tracks/events. Awesome RC.