Hi ECUPirates,
Thanks for the update and feedback … and it’s great to hear that you solved the issues you were having. It’s also great to see that you were open to a somewhat unusual foam replacement and it seems that your insights and “detective work” turned out very well.
Your feedback is also very interesting because it points to some of the more subtle differences between different foams.
While “pushback” is really a misnomer because foams “resist” and there is no “direction” to pressure (see post #2 here) … 100% natural Talalay latex is more point elastic (contours to the shape of the body) and has a higher compression modulus than polyfoam (it gets firmer faster as you sink into it more deeply) or even blended Talalay and these two properties may have been the underlying cause of the issues you were facing.
With the higher compression modulus and point elasticity it may have been more supportive (meaning more pressure) under the area where you have your back issues and this may have contributed to the pain you were having (there was more pressure or “support” under the lumbar than you were used to) and the polyfoam may be providing “lighter” support which works better for you.
The HR polyfoam is a good quality material (2.5 lbs) that will be very durable and the ILD in polyfoam would probably be reasonably close to firm Talalay (@ high 30’s ILD).
Thanks again for sharing your experiences and I’m surprised (and impressed) that you had the insight to think about giving HR polyfoam a try. Hopefully it may also help someone else that is facing the same issues as you were 
Phoenix