Do you have any information on the spring rates of the Progress anti-roll bars as a function of mounting hole used? If not, can you compare these bars' rates to those of the Hotchkis or RX-8 (red, yellow, or orange) bars?
Everything we have is in the product descriptions, including bar sizes and wall thickness when available.
Racing Beat Front MX5 Sway Bar: "25mm x 3/16-inch wall (2.67 times stiffer than the stock bar)"
Hotchkis Front MX5 Sway Bar: "Lightweight, durable and adjustable 1-1/16" (27mm) hollow, 115% stiffer than the stock sway bar with a rate of 520 lbs/in, 155% stiffer than the stock sway bar with a rate of 615 lbs/in, 205% stiffer than the stock sway bar with a rate of 735 lbs/in. "
Progress Front MX5 Sway Bar: "28.5 mm (hollow bar; 6.35mm side-wall) "
The stock RX8 bars with rubber bushings are not even on the same planet in terms of effective stiffness. Similarly, and for what it is worth, the guys at Progress and Racing Beat think that the sort of numbers that Hotchkis quotes are a complete joke. Testing the bar by itself does not really tell you much useful info because the bar is working as part of a system that includes the brackets and bushings and if you look at the Progress brackets in particular you quickly realize that all brackets are not created equal and that the Progress Bar is going to have much more effective stiffness as installed than bar of similar size using lesser brackets. Thus, all we can really say is that the Progress Bar has a much higher effective stiffness than the other two choices.