I was told the front Sonnax servos do not work well from someone that has done a lot of testing. This was an email reply when I asked why.
The problem with the Sonnax front servo is that is causes excessive 2-3 shift overlap. The way that they made the servo to increase the holding power of the front band, also causes it to release slower when 3rd gear applies. As you probably know, 727's are known to have 2-3 shift overlap problems when they are stock, the Sonnax front servo makes that problem much much worse. There is no way to modify the servo, the only thing you can do to help band aid the problem is install a low ratio lever (I would recommend a 2.9), install all 15 springs in the front clutch pack, and run as much front clutch clearance as you possibly can. I would highly recommend that you find a original early Chrysler servo with both the inner and outer springs and use that instead, you will be much better off.