Most people say/said that the 'standard' level of a group for a race bike would be/of been 105, as that is where the value per money or performance for cost starts to deminish and you only get marginal gains for considerable amount more money.

I don't think that's the case any more, I race my Sora bike. Sora doesn't hold my bike back, even after upgrading other components (contact points and wheels from stock), they still aren't holding anything back. It's the rider!

For guys racing on a budget (like myself), I'd say Sora is perfect - although I haven't used a higher mechanical groupset (only Di2 Ultegra and DA on buddies bikes).
And, an added bonus, I get this silly grin on my face when I out-sprint/climb someone with a Dura-Ace shifters that cost the same amount of my whole bike.