Not sure what you're aiming to gain by having a reservoir? The main reason for having a reservoir in a custom loop is because of bleeding and filling, when in use it serves no real purpose at all. Since you've got a closed loop system, I'm at a loss as to what benefit you're after.
More water in a loop can have a positive effect on cooling.
Without anything to dissipate heat, yeah but you have radiators. Heat is always being dissipated anyway so having more water to heat up will have a positive effect. It might not be much or worth it, but it'll certainly be measurable to some degree.
whoaaa.. why all the hate?
if it was pointless to have more liquid in the system then why would custom loops have such large reservoirs that take up space? and just because the pump has a low flow rate i dont see how adding more water will make it struggle.. wont be adding more rads
anyway, its largely down to aesthetics so i can use clear tubes with some funky dye

well closed loops have next to nothing with air bubbles to boot (you can hear the pump gurgle sometimes)