The towing capacity is largely due in part to the rear end. You can have a rear differential set up for towing or for speed. That's why the Jeep Grand Cherokee can tow more with the 5.7L than it can with the 6.1L. However, the 6.1L SRT-8 Grand Cherokee can go 0-60 in under 5 seconds.

Possible upgrades you could do are add a cold air intake, performance exhaust, look into an ECU reflash, and better headers just to name a few things you could do. Really they're two different animals though. The 5.7L you have is set up to tow, you would do completely different types of mods to it to get better performance out of it than you would the 6.1L on your Challenger.