Even if your hubs are wide enough and you do upgrade to ST-5800, they're not compatible with the Sora derailers. It's true that 11 speed needs a wider freehub, but that's not necessarily a good reason to get 10 speed parts over 11 speed parts if you plan on eventually upgrading wheels. You can run a 11 minus 1 cassette with a couple shims on a 10 speed freehub, similar to Sheldon's procedure for "9 of 10 on 7" for short 7 speed freehubs. It's not a particularly useful way to upgrade if you intend on staying 10 speeds, but it can help you carry over parts when upgrading incrementally.

Incidentally, my opinion is if any part of the bike is holding you back, it's your fit, tires, or having inappropriate gearing choices for your strength and terrain.