I have the Progress MX5 front bar with the stock rear bar in use with 250 and 200 pound springs on FCM coilovers. I think you can expect roughly 50% reduction in roll from the Progress bars (I did the sway last). In some ways, I wish I had gone with the Progress springs instead of coil overs, since I value street comfort over the advantages of higher-rate springs (less dive and squat, etc).
I'm going to redirect slightly... You mentioned the Progress RX8 bars for even flatter cornering - what would you recommend for my setup? I generally like the current balance, though sometimes it feels like it has too much oversteer. Would you use all RX8 bars or the front mixed with the MX5 rear bar? Or some other combination?
2010 Grey MX-5 GT, 6-speed Manual, Sport Package, Roadstersport Street Single w/Baffle, FCM Coilovers - "GT" setup with KBO & Ripple Reducer, Enkei 17x7.5 wheels, Goodwin Underbody Braces, Cravenspeed Shift Knob