OK... to add on to what the others have said....
1) Pay for a good fit! top performance upgrade you can get.(might include a stem or saddle change)
2) Decent tires/wheels.
3) Good helmet (well ventilated, comfortable).
Things to NOT change
1) 105 componetry - unless you want to go to DI2, 105 is imperceptibly slower on shifts than ultegra or dura-ace
2) Handlebars - you've already got a decent set with the IsoCore bars. You won't get much more comfort and no performance gains by swapping unless they are the wrong size for you.
Other "cheap" upgrades that can make a difference - good bar tape (i prefer SupaCaz myself), decent gloves and clothing. You've got a decent bike already, make it as comfortable as your budget allows and go get riding.