A few minutes with Excel and a beer:
Numbers are final drive ratios as compared to crankshaft speed.
4l60E has the same ratios as the 700-R4.
The rear end ratio is on the left axis.
As you can see, the TH200-4R's first and second gears are deeper and provide more mechanical advantage across the board than the Th-400. So much so that at 3.23 gear combined with a TH200-4R is still going to be better than a TH-400 with the stock 3.42s.
All things being equal, a car will be quicker with a 200-4R than a th-400. The ability to shift into fourth means you'll be faster, too. Ultimately. A lower rear gear numerically increases maximum speed, which you need to pay attention to since your top gear is 1:1.
For each inch in tire diameter increase, you'll gain 3.14 inches of travel per tire revolution. At 6000RPM, your top speeds for various gearings (assuming 1:1 high gear in the gearbox and no slip in the convertor) are:
Of course, this model is oversimplified, and doesn't look at RPM drop between gears, extra weight, etc. The actual answer, as in all things, ends up being "it depends."
But if you're trapping close to 146, then a 30" tire might be the next step.
If you're not trapping close to 146, you may have other things to look at. Or not. It depends.
