I doubt the 17t motor you mention is that much faster than a Saturn 20. The Saturn 20 is pretty close to the limit of the car in my opinion, it's a proven performer and I would recommend it over the Etronix motor.
Cornering style in a Mini is very different to a rear-wheel drive car. You don't need to manhandle them through tight corners because they are small and will turn tight quite easily. What you do need to do is to get any braking done in a straight line before the corner, turn it in, and get on the power relatively early to transfer the weight back to the rear tyres, this will stop the back end from looping around.
Tyres and the front diff setting are really, really important important in a Mini. A tighter diff than the kit gear diff makes the car a lot easier to drive, it stops the front end from hooking in the corners and gives you more consistent traction out of the corners. The Tamiya TA03 diff is probably the easiest to use - just build it and fit it and the car will be greatly improved. A lot of people use the 3Racing gear diff as well - it's an oil-filled unit so has a different feel in the car to the ball diff. The 3Racing diff needs a lot of running in before use because the o-ring seals are too stiff, but once run in, something around #50k to #100k oil should be a good starting point.
Shocks and springs are not very important, but the car runs a lot better with oil filled shocks (preferably Tamiya) and short Tamiya springs.
My
blog is full of guff on setting up a Mini, I've dedicated most of the last two years to them! Mine runs like a 4wd tourer, very easy to drive fast.