I think the Birds suspension deal is over priced personally. I read mxed reviews too which dissuaded me.
The main one seemed to be the difference was not as great as expected.
The oem setup is actually pretty good and quite firm already - its the rear bar that is too weak for a 'drivers car'.
I only replaced mine as I had a failed front strut anyhow and some Hartge(revalved Bilstein) dampers came up for silly money on here.
But lets be honest, if you want to ride a 'magic carpet' you dont want to be messing with high rate springs et al.
It's all a compromise and if you want to pull high G corners, then the ride isn't ever going to be smooth in a 1.6tonne chassis
I sourced my own bars (H&R as a pair) fitted the front myself and got the rears done along with the diff by Birds.
Even without any other suspension mods, try the anti-roll bars with a diff FIRST as they make a big difference and may be all you want from a road/mild trackday car.
It's no real extra cost to try it this way, the only labour difference being bottom suspension arm bolts out and lower damper nuts off (30min tops).
So, get some bars yourself (H&R remarkable similar dimensions to Hartge) and then take it to Birds to do the LSD with M3 subframe bushes and fit the rear bar at the same time.
Any garage at all, even a home mechanic, can fit the front bar easily enough.
See how you like this before moving down the full suspension strut or coilover mod route.
My 2p
Steve