Is there something the 105 group isn't doing for you? 105 is a good group and shifts great when adjusted properly. One reason to drop the cash for Force is that you tried SRAM at a shop and just fell in love with it. If so, more power to you.

Otherwise, there are other things you could upgrade. I would focus on the contact points: saddle, shoes, handlebars/stem. Then make sure you have any gear you need to ride in the conditions you ride in. Shorts/bibs, gloves, etc... I first get the gear to make the body comfortable, then I worry about gear for the bike