I'm playing with drawing too on my Surface pro 2 and from my experience the program used for drawing seems to make a lot of difference. So it's worth experimenting a bit.
For me using Clip Studio Paint seems best so far, as it has dedicated tablet mode UI and is responsive enough, thought 1200 dpi A3 will might be too much for it at times.
But trying to draw on an 1200 dpi A3 canvas (unsurprisingly) doesn't really work very well. But if I were to use it professionally, some of the things I'd want to do was draw high resolution A2 to A0 prints.
I've yet to play much with photoshop, so I'm not sure how well it handles a modest 20 megapixel image.