[MENTION=202156]BuffaloBill[/MENTION]...well I can tell you weight wise I am not carrying too much, but I use both my panniers. In one I carry my lunch, cause eating out everyday get expensive, and the other holds a change of clothes (much lighter now that tht can consist of shorts and a polo) and my size 13 shoes...which can be a pain to fit in somewhere else. I do keep a pair at work, but those are more for if I forget to pack my shoes. Being a very casual work environment, I don't have a designated pair of work shoes I want to leave there.
Add in some basic tools (more than usual right now because I am still dialing in my fit and everything is bolt on), repair kit with new tube, tire boot, levers and an allen key, and maybe all in that's 10 pounds of weight, probably less. Less on the way home due to lunch being in my belly.
It makes you strong! I've noticed on my recreation rides that I have gotten stronger than some of my friends, especially going uphill whilst not lugging a bunch of stuff.

Anyways, I don't usually have spirited commutes. I like to keep them around 14 moh averages or I find I burn myself out