generally speaking the reservoir is not gonna effect temps. the water is not in the reservoir long enough for the heat to dissipate, especially if it's just a plain old acrylic res.
if you don't want to use a res and free up some space then just get a T line, stick a fillport on the end and that's it

just stick on of those on the end of the T line and it will allow you to fill and close off the loop, a lot more work than using a res but a lot smaller