Hi hmorneau,
The Pure Latex Bliss latex foundation uses 4" of latex on top of the foundation.
It will change the “feel” and performance of the mattress and generally make the mattress feel softer. Whether or not the extra cost was justified would depend on whether any improvement in terms of PPP and the feel and performance of the mattress was worth it to you.
Their regular foundations aren’t the best in terms of their structure and quality (see post #5 here for a picture) and I’ve talked with a number of retailers that wish that Pure Latex Bliss would supply a better quality foundation with such a high quality mattress.
Phoenix