105 is significantly nicer to use than Sora. If you're buying a new bike from a shop, and the cassette is a bit close ratio for your taste, any shop worth its salt will swap it out for one to suit your needs (after all they can then sell the one they've just removed to someone else)... If not, then an 11-28 105 Cassette is only about £35 online these days anyway.
If you change to a 28 cassette be careful with chain length and rear mech too. Should really have a medium cage mech for a 28 cassette with a compact chain set.
Total cobblers. On Shimano these days you can run up to a 30T cog on a short cage mech with a compact chainset. At the very worst you might have to add a link to the chain going from a 25T top cog to a 28T, but 99 times out of 100 you won't even have to do this.
FWIW I wouldn't even consider buying a carbon bike with anything less than 105 on it. I don't know why some manufacturers seem hell bent on putting Sora on Carbon frames just to get the bike to the £1k price point! There's so many good ally bikes these days that are no heavier than cheap carbon (and often better quality) that come with full 105 kit for the £1k price point after all.