Swapping to better tyres with a lower rolling resistance will make more of a difference to speed than a new groupset, and to an extent, new wheels. And it's far cheaper. Working on the engine (you) will make the biggest difference. I can achieve similar speeds on my 105 equipped aero bike that weighs about 7.5kgs and my tiagra equipped CX bike weighing in at closer to 10.5kgs. The CX bike just takes a little bit more effort, but it's not due to it's weight, groupset, or wheels. It's more to due with geometry and tyre choice. And I'm not that strong of a rider, just your average mid-pack club rider. Now neither of those bikes can compete with my TT bike when it comes to pure speed. But again, it's not due to groupset or wheel choice, but the fact it's a TT bike, and made for speed.