You're over thinking this one.The tire size (width/height) has very little to do w/ rims width, within reason. It has more to do w/ clearance on the machine.
I am running four sizes larger on both front, and rear on one bike, and 2 sizes larger on the rear of another with one size smaller on the front.
It all comes down to geometry, and how the frame is designed. My bike w/ the two larger tires rides 1000% better than OEM, and my mixed size bike rides no hands all day long (home made "O" ring cruise control). Fast corners are no good for this bike but I just take them slower.
What it comes down to is balance (geometry). Taller tires effect the balance. Wider tires do not.
If your bike tracks too hard in corners go taller in the front or shorter in the rear If it under steers go shorter in the front or taller in the rear.
No one but you can determine what size is best for your bike. I have a pile of tires that did not work out as I had hoped. Luckily I can sell them to other people as they are needed so I lost nothing by experimenting.
What I can say for sure is wider is always better. You get a larger contact area, therefore better grip.
On a PS note... The OEM Chen Shin tires are junk. Anything you get in a name brand will be night, and day better.
I am not over thinking this. It is a well Known fact that to big of a tire will mess up the handling of ANY bike. the question is, how big is too big. When you put a tire on a rim that is too narrow, the tire will end up in a shape not intended by the designer and you will get funky handling. You admitted that your bike doesn't handle fast corners well. My Super 8 with stock size tires handles like it's on rails at any speed. It's only real limit is that it runs out of cornering clearance before it runs out of grip.
I would love to replace the rear Chen Chin with a better tire but have yet to find one that is close to the right size. You just confirmed for me that the larger tires don't handle well: "Fast corners are no good for this bike but I just take them slower." My only real complaint with the Chen Shin is that it doesn't work that well when it's wet. The front Chen Shin worked pretty well too but I replaced it with a City Grip. The city grip made the steering a bit heavier but now that I have gotten used to it I don't even notice it.
BTW, it's a well know fact that wide tires tend to make bikes stand up in curves and also make them more likely to stand up while braking in curves. Wider tires also tend to make the steering heavier and slower. Go read some magazine tests on bikes with really wide tires and you will see what I am talking about.
As for going with a tire like a 140/60 or 140/70 on the rear of the sport city, I am still waiting for someone to tell me how that handles. What exactly are you running on your Super 8?
As for the Stock tires being junk, they last a loooong time and have great feel. It is a well known fact that they suck in the rain. I got 16K out of the OEM front tire and am at 10K on my second rear tire.
Also, I have ridden my Super 8 in a wide variety of conditions, not just around town. Overall the stock tires worked pretty well.
A few of the conditions I have ridden in with stock tires:
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