Pete, if you don't mind me asking..what color did you go for and how is the interior quality...Hello SR45. As a new CX3 owner I will try to answer your 7 questions.
1 Info system. Only used the radio and iPod things so far - both really good sound quality. CD player is redundant. Do not use streaming I over Internet. Still a lot to learn though.
2 Sound insulation. This is subjective, but as I said in another post we are happy with the quiet cabin. Have not driven on rough dirt roads, but poorer quality hardtop no problem.
3 Loose knobs etc. none at all, it is all very tight.
4. Fit and finish. Have not found any blemishes or fit problems anywhere. It all looks classy.
5 Engine sound under acceleration. There is a "growl", but I think it sounds good and is not intrusive.
6 leg room. Not sure what measurements you need. With driver's seat fully forward: 15.5 inches from leading edge of front seat to brake pedal and 11.5 inches from leading edge of rear seat to back of driver's seat.
With driver's seat fully back: 22.5 inches from leading edge of front seat to brake pedal and 4.5 inches from leading edge of back seat to back of driver's seat. Maybe you could compare those measurements to your/ another car.
7. Reversing camera. Yes got this, came with the spec we bought. Ladder grid in the screen is useful and the car beeps a lot as I back out of the garage past the pillars.
Also, the car's computer is telling me I am averaging 8 litres per 100 kms (no idea of mpg). Have not done the second fill up yet to calc it myself.
Hope all this helps.