Beginning with your Masi frame the best place to shave grams and get the biggest benefit would be the wheels. Throw on some aluminum Easton, American Classic or many others will get you some instant speed. As far as shaving some weight on the bike, maybe carbon cranks with external bottom bracket will shed some grams off the bike. In general, you are never going to get the lightest bike possible with a steel frame. Just lighten it up some by exchanging parts.
I know what you mean about the ride of a steel bike, I love my Jamis mtn bike built with Reynolds 853.
Some of the new carbon frames are the cats meow, huge bottom bracket area to mitigate flex, wispy chain and seat stays to cut out road imperfections. Huge steerer tubes to counter flex in the fork and generally just being a whole lot lighter than a steel frame.
You are probably running 9 spd on the 105. All the new stuff comes with 10 spd. Also, living in Portland I am sure that there is some climbing involved, drop down to a compact crank and get some lower gearing for the climbs. I hear that Ultegra is on the same quality as Durace just a slight difference in weight between the two.
You can find some great deals on Chinarellos (China produced carbon frames) with a complete Ultegra build for a fairly reasonable price, i.e. Stradalli, Sette, Fujis and many others are having frames produced in China for incredible savings for the mfrs and the consumer.
If you have any questions bounce em off me. You might also want to hit up 'swissracer' on this site. He was an ex roadie pro, he has tons of info to share also.