I do approx 16K miles per year in my 2018 RC300h F-Sport I average around 42MPG doing 25mile each way commutes mainly in 'Comfort' mode, don't use Eco mode as that is worse on fuel economy and slower to pick up on acceleration. The best speed for economy is 73MPH indicated which is actually 70-71, my speedo reads 2-3MPH fast, I average 550mile to the tank and on premium fuel normally get 2MPG better than on regular but I normally only use premium every third fill. Remember the RC is a GT not a sports car but in Sport + mode it can bring a smile and flick it into that mode for a quick overtake.
Recently I fitted a custom axle back performance exhaust and a K&N air filter, the car does seem to have found a few more horses and the traction control does also cut in a lot more than it used to (rear tyres and in good condition, don't know for how long though!), in sport + mode with hard acceleration I can even now hear induction noise and also the engine now revs more freely. It was noticed and mentioned by the mechanics fitting the exhaust that the standard one has a restrictor just before the final back box, that restriction is no long there on mine.
I have the standard audio which if listening to the radio is fine and at 'normal' volumes is ok but turn it up to ear bleeding levels and it distorts and gets tinny, I keep it down and am happy even on motorways. Standard SAT nav is fine I've had worse, only issue I have is you need to know the street name not just the postcode to find a route which doesn't always work when going to events located in the middle of the country.