Upgrading your internal hard drive to an SSD (preferably a Samsung PRO series) will increase performance noticeably, and may squeeze out a little extra battery life as well as lighten your laptop a few ounces.
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Replacing your optical drive with another drive will give you extra internal space. I am not sure whether or not this is moot, as you stated you have an external drive already.
Upgrading your internal hard drive to an SSD (preferably a Samsung PRO series) will increase performance noticeably, and may squeeze out a little extra battery life as well as lighten your laptop a few ounces.