Agree 100% . Once you start racing properly you will end up with enough kit to upgrade the winter bike but still wont bother doing it .Winter bike? Save your money and leave it on 8 speed.
https://www.merlincycles.com/shimano-105-5800-road-bike-gear-kit-85057.html
Don't know if this any good for you
Why not just get an 11-28? Don't they exist?
http://www.ribblecycles.co.uk/shima...BP&gclid=CPvWybSfkNACFRWNGwodDhgAVw#pid=19494