How to gain more top end without sacrificing low end
 

Supercharge it.

With most NA mods you're going to give up a little bit of low-end torque for high-end HP. For instance, headers free up high-end HP but the lower back pressure means a little less low-end torque. Not much but a small amount.

Could look at something like an underdrive pulley to free up some HP. Should be fine as long as you don't have an aftermarket stereo or other items putting a big drain on the electrical system. Make sure to go with a reputable one (not the eBay special) as they need to be properly balanced or they can cause other issues.

Air intake like the Icebox will improve high-end power.

Headers all the way! I only noticed a slight drop in low end torque when I put mine in...but the gains up top were noticeable. I hit the Rev limiter by accident the first few times I punched the gas pedal.