Upgrading Cannondale Synapse Flat bar 105

[kashiy]

I have a Cannondale Synapse Flat bar 105 and have been riding it to work over the last 4 months. Have to say its been an absolute adrenaline rushing pleasure ! Now I,m at the point of considering whether to upgrade the bike ie new wheels, shift to drop down handlebars etc. But what I,d like to know before I do this is if the Synapse Flat Bar frame is worth upgrading ? Or shall I just keep it as it is, use it over winter , and get a new upgradeable model in Spring?