well the T line people have been heard from
and for the most part having a res will not lower your working temps at all, if you rad is working efficently, and your water exit temp is at or near room temp.
but if you rad can not handel the load, either because it is too small, or if you have a air flow problem on it, or it does not have enough water flow (or actualy too much) and the water is not in the rad enough time for it to cool off. If any of the problems i have shows are happening, and if you res is made of a heat conducting product, such as copper or alu, or any metal for that matter. Then and only then will having a res lower your max load temp.
and keep in mind that the larger the res the more water that is in the system, the longer it will take for you system to get up to max temp, but with my 2 systems, one with a T and one with a 1 gallon res, the res only slows down the temp rise for a few hours, it will get to the same temp as the other setup, but with the res it is easier to fill and drain, and it bleeds out all of the air in a matter of seconds with out having to pulse the power to the pump, like i do with the t set up.
hope this helps