I don't know if this is true or not, I did the B&M many years ago. What I was told is that the B&M shift firmness was related to how hard you hit the gas, so if you were light on the throttle the shifts would not be as firm in comparison to WOT. With the Jmod the shifts are always firm. I was told this was due to the progressive rate springs in the B&M kit. Again, not sure if true or not, but I sold my Mark VIII with 168k and 100k+ had the B&M kit in the trans and the trans was still shifting great. Also I had an auxiliary trans cooler.