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Here's what I mean:
For example of an 8.5" wheel, the first chart I find (can't confirm accuracy of this) lists tire widths from 225-255, with 'ideal' at 235-45. A 225-235 will give the best steering response, because the sidewall is most taut and the tire has little ability to squirm. A 245-55 will give a softer ride, but also mushier steering response, because the sidewall is rounder and has more room to flex or roll over. These are not going to be dramatic differences with a constant wheel size (factory is 7.5" w/ 225 and 8" w/ 245).
Aspect ratio will be determined by the ultimate width and diameter, assuming you need to keep total diameter at/near stock (highly recommended). Wheel diameter is usually bigger-is-worse, except for clearing better brakes (or looks) - the unsprung weight negates any advantage of low-aspect stiffer sidewalls).
In general I'd avoid extremes, especially for really hard use or on track. Personally I hold with the belief that section width is only really relevant in two situations - track use, or very wet/snowy weather (other opinions are out there). The contact patch area is the same regardless of width, but it changes shape - narrower is better in wet/snow (less hydroplane effect) and wider tires are better on track b/c stress and heat are distributed over a larger area (and sidewall rolling affects less of the patch). Even for track days I don't see the need to cram the most rubber in. Check out most track instructors and they will likely tell you that you are better off learning on less aggressive tires, since tires can crutch skill deficits, or uncover them at much higher speeds; also you aren't running the tires for a full stint in a race, so they have time to cool down (so why not stay w/ 225's and assure no rub). This is one of those areas that is heavily polluted by cosmetics and advertising hype that bigger is better for both diameter and width. There is tremendous placebo-effect when you spend 2k on a setup, but I'm suspicious that few would really notice a change of 10-20mm in section width on the street (or a novice on track).
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