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Yeah man. Anytime that we put big wheels and tires on a little car, we're going to get some of these symptoms. There are a variety of explanations - most of which, you can't really fix (short of buying different wheels and tires).
I would say that these things are beyond your control:
1. The wider tire has a larger contact patch with the road. This is good from the standpoint of providing extra traction and thus extra grip in the corners. But it also has extra traction over uneven surfaces too. So it gets pulled and pushed by the road. And yes, you can feel this in the steering wheel.
2. The narrow aspect ratio of the 215/45 means less tire damping effect - (you can also think of this as a higher spring rate from the tire or less tire shock absorption) - which drives more force from bumps and road imperfections through the suspension and into the rest of the car. Once again, you can feel this from inside the car.
3. I don't know what brand/model of wheel you have. But the chances are good that the weight of your 17 inch wheel/tire combination is significantly heavier than the stock wheel and tire combination - which means that it is carrying a lot more inertia. And a wheel with more inertia is harder to control - which means that the extra inertia can more easily overwhelm the suspension. (Or said another way, the suspension has to do much more work to control the extra inertia of the wheel. At some point, it is unable to keep up the extra demands caused by the extra inertia.)
There is not much you can do about these things, other than replace your wheels and tires with a new set that is smaller and lighter.
That being said, I will throw out a few possibilities:
1. Check your tire pressures. If your pressure is too low (or too high), then the tire is not performing to its original specification. I can always tell when I have a low tire because my car starts to pull.
2. Make sure that your tires are not directional and installed on the wrong side. Some tires are built with a left side and a right side. So when mounted, 2 should be mounted with the left side out and 2 should be mounted with the right side out. And when installed, you should put the lefts on the left side of the car and the rights on the right side of the car. I accidentally flip-flopped these once and my SL2 was all over the road.
3. You can mitigate some of the effects of the heavier wheel/tire overwhelming the suspension by bracing the front strut towers. This does not change the fact that the heavier corners are directing more force through the suspension, but it will reduce the flex and compliance that results - and makes it easier for the rest of the chassis to do its job.
Lastly, don't forget that a 10+ year old car is showing its age. So you will get extra slop and compliance from worn bushings, old shocks, loose fittings and connections, and so forth. So again, any extra compliance makes it harder for the suspension to do its job. I know this is a pain and an expense (and might not be worth it on a 10+ year old Saturn), but in an ideal world, it would be great to install new shocks, new bushings, and new suspension fasteners that are all torqued to spec.
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