Originally Posted by
RWTD
For me it depends on the biomechanical position my back is in and whether that tends to be a strong or weak position to support weight.On a road bike I am aften in a 45% or less riding position with my back rounded at times(particularly in the drops) so my back is often in a weak position to support added weight so I rarely wear a backpack but like some others have noted bungy it to the rear rack.On mtn bike tours where I'm usually 45 % or above I almost always wear a pack on tour or day ride and sometimes quite heavy.But I always keep my back flat/flexed even leaning foward on the flat bars.From what I can tell you are talking about a performance road bike so I would be cautious adding weight to your back .It can be done with lite weight and it is convenent like you say for off bike but on my road bikes I always eventually just strap it to the rear rack and access it off bike from there(if a day pack/lite weight pack).
Interesting that you find it more of a problem when using lower bars. My thinking was the opposite. I find it more annoying when more upright since it hangs on my shoulders. With the amount of weight I am proposing I think it will be fine either way, but I ride with bars 4-5" below the saddle so the more upright posture doesn't factor in. I figure that what I expect to have in the backpack is less than I have at times jammed in jersey pockets (at times I have carried a couple liters of water plus some other stuff there).