You can calculate the difference in spring rate of a torsion bar compared to stock by taking the ratio of the diameters raised to the 4th power (or square it twice) So your 28mm bars are (28/22)^4 = 2.6 times stiffer. This is a HUGE step, so I don't recommend it unless you also increase the front spring rate by a similar amount. Since front springs are very limited, I think you should stay conservative on the rear bars.
Stock bars exist in 23 (or 23.5?) from a Type 3 (Hans?). Early Porsche 944s and 924s have 24 or 25mm bars (and more) I think Geoff knows what's available from 944s. There are a lot of different sizes in Porsches. They had options of sport suspension, turbo, S, etc.
You can also look at Porsches for perspective. Full race for a late 911 is a 28mm bar. They have a LOT more weight to control, and a lot more hp.
I would go with the Type 3 bars or a 24 bar at the most. Then upgrade your shocks. Bilstein or Koni are adjustable, so you can play with their settings to find what you like. Konis are $$$.
By far, the biggest contributor to great cornering is wheels and tires. The more the better.
I believe the front bar is 19mm stock. You can get a 7/8", and maybe more.
SAW rear bars are 3/4". If you decide you don't need such a big rear bar, you can bolt on the stock bar from a 924 or early 944. I think they are 12 or 15mm, but don't quote me on that.