Stephane,
I'm the builder of ACSS.
Yes, you're right. ACSS is designed for HEAD only, unlike the BTS which was designed for Raichle / Deeluxe boots.
With ACSS, nothing is overpassing behind the heel, and this is better for laid backside turn with low rear angle.
Right also :(
BTS is made on CNC machine with high end surface treatment, while I do ACSS myself with a few manual tools ...
As mentionned above, for both system, it's absolutly necessary to customize the shell to take advantage of the spring effect.
You can download here a document with all cutting instructions.
Arnaud